I'm thinking of starting a Dungeon World campaign. It seems from the experience system that levelling can be pretty quick. How many sessions would you expect to play before characters start retiring etc?
[RPG] Dungeon World campaign length – how long to reach level 10
Nothing in Dungeon World is a straight conversion of D&D – everything is re-imagined. Even the base classes provided can't be used to convert a D&D character straight across (for example, in stock DW there's no way you can make a Dwarven Druid, while you can easily do so in D&D 3.x without creating a house rules). A straight conversion of new material is never going to be simple. To convert new material, you have to re-imagine it fresh, with the aim of capturing the flavour and style of the material instead of the raw abilities.
For complex or powerful races, you are actually better off creating a new playbook for the race, with a few optional abilities to pick depending on what class they are. So instead of picking up the Fighter book and choosing a Fighter-class Drow racial ability, you pick up the Drow book and pick a Drow-race Fighter class ability. Doing it this way you get a very Dungeon World–style character: they're the one Drow (or Warforged, or Dragonmarked, or …) in the party, and their abilities have a flavour and mix that is unique to them.
You still have the challenge of balancing a new playbook, but you avoid the mess that can easily result from hacking up and rebalancing the existing class books.
(First, I'm not a lawyer, this isn't legal advice, retain a lawyer if you have any doubts.)
Referencing things has nothing to do with copyright law
Let's get one thing out of the way: Dungeon World's references to bits of pop culture and literature and stuff is in no way a violation of copyright, trademark, or patents. Trademark and patent law don't even remotely apply, and copyright prevents copying and transforming a copy, not plain referencing. Those are the three prongs of IP law, the entirely of what creators have to worry about, and none are relevant to referencing pop culture. This will probably make the rest of this stuff make more sense.
You can make stuff for Dungeon World. Please do!
Dungeon World is licensed under the Creative Commons Attribution 3.0 Unported License. That means you can make anything you want using even its text, and you can sell it so long as you give attribution to Sage and Adam as the creators of what you're using. You could even just copy the, say, Druid class wholesale, call it "The Shaman" without changing anything else, stick an attribution line at the bottom, and put it up on RPGNow for $5 and that's legal. See "How do I properly attribute material offered under a Creative Commons license?" for details on how to correctly attribute Dungeon World as the basis of your work, if you end up using any of its text (like, say, borrowing a Wizard move whole).
But for just a class, you don't even need that. Publishing a class that's compatible requires no use of copyrighted material from the original, so you don't even need the CC license in that case. You just publish.'
Even saying "This is compatible with Dungeon World" is legal: that's called "nominative use", and the use of a trademark to indicate compatibility is a protected right that you have. TSR went to court over this ages ago, and lost hard, so it's even a fact that's been tested in the RPG world. Kenzer Co openly publishes material for D&D without using the OGL, because they aren't copying any text and it's legal to say "This works with Dungeons & Dragons" without getting WotC's permission.
Besides which, Sage and Adam encourage people to make stuff for Dungeon World, an have provided a "For Use With Dungeon World" kit that you can (optionally) use if you want a recognisable shared graphic.
Lots of people have already created new classes for use with Dungeon World and are selling them just fine. Not only are you in the legal clear several ways from Sunday, Sage and Adam aren't interested in suppressing that kind of thing.
You should be paying more attention to the actual threat
All that said, you do have to be careful basing a class on the Witcher. File the serial numbers off, make sure that people can recognise it but you're not using any words, names, images, visual character designs, or anything else that is owned by Atari, and you should be okay. If you're planning on making any money off it, you may want to research your legal rights and obligations here. You can produce something based on someone else's work, but it has to be pretty substantially separated and only recognisable by people who really get the references.
A DW class PDF for sale on RPGNow based on the Witcher is highly, highly unlikely to draw Atari's attention, but you never know. They've sued people into bankruptcy for less. Your safest best is to not make it obvious that it is a Witcher, because if you use Atari's trademark to draw attention to your class, if you advertise "You can play the Witcher in Dungeon World!" as a feature of what you're selling, you've violated their trademark and you are—if they notice you—potentially in serious trouble. At best they send a Cease & Desist to RPGNow and they pull your PDF. Worse, RPGNow closes your account. Worst, Atari goes for you directly.
So don't get noticed, and don't do anything that will make anyone notice that your class is based on the Witcher until they've already bought it, and you should be in the clear.
Of course, if you're not selling it, then you can probably get away with saying you've made a Witcher class and here's the PDF, since there's a lot of precedent for ignoring fan-made stuff that's not for profit, but you still have to watch out, because you're on the wrong side of the law then, and you're only OK so long as the company a) doesn't notice you and b) turns a benevolently blind eye.
This question really heavily depends on your groups style of play, frequency of moves and duration and frequency of sessions.
Frequency of sessions: the more often you play and the shorter your sessions are, the more often you will use the End of Session move and thus earn XP at a faster rate.
Frequency of moves: the more often a move involving a roll is triggered, the more often a 6- will be rolled, thus marking XP. The frequency of moves strongly depends on how the players like to play. A more adventurous, aggressive, less cautious group will trigger moves more often than a group that is more into roleplay and making eloquent plans to deal with situations (thus avoiding lots of hack&slashing) .
Style of play: min/maxing players will level at a much slower rate than players who don't. They will optimise their stats, thus getting a +3 mod on their most used rolls quickly. They will use the fiction to their advantage and use solutions that involve moves where they have a high modifier.
That said, a campaign does not end with the PCs reaching "level 11" (there is no 11th level). In Dungeon World, you are encouraged to build your campaign around the characters, and change the world just as they do. Even if the rules state that at when they would level up from 10 to 11, you must choose one of the options presented, the character does not just vanish from the world. It migh settle down somewhere with a family. Become barkeeper in a local tavern. Two of the options even let them stay with the party.
On a closing note, my group currently has played 8 sessions at 4-6 hours each and we are around levels 5-7.