How much time do I have to spend in hell to earn the “Medium Well” achievement in Super Meat Boy

Bandage girl has a random chance to glitch every time you spawn on a level. She can glitch in any level from 1 to 20 for each of the worlds 1 to 6, whether you're in the light or dark half of a world. There is exactly one negative level for every world.

To unlock level -1 for a world, pick a level in the world that is short and relatively easy. Below are good examples (in the light world) including a recommended character:

• 1-3 (The Gap) using anyone
• 2-10 (Johnny's Cage) using Jill (if you don't have Jill, see this video)
• 3-6 (The Shaft) using anyone
• 4-6 (Leviathan) using anyone
• 5-10 (Grinding Mill) using anyone
• 6-2 (Schism) using anyone

Then:

1. Look at or get to bandage girl and see if she is glitching. She will be spinning and making a distinctive pinging sound. I recommended levels where you can (mostly) see her from spawn.
2. If she is glitching then touch her to unlock level -1. Otherwise, kill yourself, respawn, and go back to step 1.
3. To play the level you just unlocked, go the light half of the world you unlocked it in. It will be available to play as a yellow-colored level beside level 1.

If you've collected twenty bandages, you've already unlocked it. You may have gotten a message after you collected your twentieth bandage that said something like, "You have unlocked Teh Internets", "You have unlocked English", or (most recently) "You have unlocked something locked."

The locked question mark is a new zone called "Super Meat World." It's intended to be a never-ending stream of new levels for the game. An update to the game in mid-January will unlock it, along with an extremely capable level editor, and an online level sharing website. Users will be able to create new levels and upload them to be rated and played by others.

From the developer's blog:

• Super Meat World will act as a Hub for full chapters hand picked by Team Meat.
• All chapters uploaded to Xbla after this update will be featured here.
• Select chapters will also be uploaded to Xblas "internets" chapter as well.
• Many selected chapters will be character specific and created by designers like Matt Thorson (jumper / give up robot), Gaijin Games (bit.trip series), Michael "Kayin" O'Reilly (I wanna be the guy) and many more.

But! by far the coolest aspect of Super Meat World is your ability to "Enter the Unknown". Enter the unknown is a Pc specific feature that grabs random user made levels from the level portal and arranges them in order of difficulty.

• When Players enter the unknown they will choose what difficulty they want to play.
• The unknown will only feature levels ranked 3+ on over all fun factor.
• Every time you play will be a totally new set of levels, endless content.

Here's a link to the source, with more info