In each area of the game (Let's refer to them by the final bosses) there is a general Soul Memory level that people are when they reach the boss. For example, on the Last Giant route, players are generally Soul Memory level <10,000. But from there, the choices are very open as to where a player will go, (Lost Sinner, Old Dragonslayer, The Rotten).
So what Soul Memory level should I be, in which area, for Coop PvE?
Best Answer
In general there are so many players, that will choose to do more or less the same as you, that statistical your soul memory in combination with a specific area has little relevance. There will always be someone to Co-op with. Especially in NG.
However, to maximize your chances the best way would be to follow the progression/paths from some of the big internet guides, such as:
http://www.ign.com/wikis/dark-souls-2
http://darksouls2.wikidot.com/areas
If you follow these routes, you will more or less be playing like hundred of thousands of other players, and you should have lots of opportunities to Co-Op.