The time for getting the achievement (Medium-Rare) is about 495 seconds. My time is 475.something seconds. There are now four other achievements, Rare, Medium, Medium-Well, and Well Done. I don't know the times for those, though I know that I have the Medium-Well achievement.
I'm going to take a wild guess here and say that
Rare = 496+ seconds
Medium-Rare = 495 seconds
Medium = 485 seconds
Medium-Well = 475 seconds
Well Done = 465 seconds or less
Or it could be the other way around, whereas you either put the order of the names backwards, or the times backwards. Now, I have 475 seconds and the Medium Well achievement, so if it was the way I am describing now, then it might be something like:
Well Done = 500+ sec
Medium-Well = 499-475 sec
Medium = 474-450 sec
Medium-Rare = 449-425 sec
Rare = 424 sec or less
The second way makes more sense to me because it takes longer to cook a steak to well done than it does for it to be medium-rare.
But, I really don't know. If Tommy or Edmund posted here, then it would help.
EDIT: The time for the original Medium-Rare achievement is EXACTLY 500 seconds.
OLOOKY. Rare is for Forest. Medium-Rare is for Hospital. Medium is for Salt Factory. Medium-Well is for Hell. Well-Done is for Rapture. And you have to "Spend as little time as possible" in each of those chapters to get the achievements. That is,if that site is correct.