just another quick edit (i hate doing those:p) but just an FYI that Woody is xx was in fact the first person to post a soloution and his method did work i just happened to find an easier one so for the sake of giving credit where it's due i have given it as such :), now for an easier way all you have to do is take control of a rat and then crawl inside of a vent, you won't shift back until you're back out of the vent so just stay in there for 3 minutes and crawl back out and you're all set.
not usually one to edit things but people seem to be confused so i'll clarify that you need to change back into Corvo for the achievment to pop, so set a timer for 3 minutes when you're in the vent then crawl out and achievment done.
hopefully that is all the editing that need be done, i didn't bother with much detail because honestly i assumed it was easy enough to figure out once the basics of it were sorted out