This is coming from a total top, so take it with a grain of salt. But I've fucked a lot of guys, and noticed how some of my boyfriends cleaned out. And had some bad experiences with ones that didn't do it right. There are two ways to clean out, just the rectum, or the whole works. If you've got a good diet, the first is fast. The second takes hours. If you're getting fisted, you need to do the second. If you're getting fucked, you may be able to get away just cleaning out the rectum. You can do this on the toilet. With a bulb, a disposable douche/enema, or one of the douche nozels that screws onto a water bottle. But don't use too much water at once, because if you get it around the bend, you will be leaking nasty water forever. You should be able to clean just the rectum quickly on the toilet, despite the roommates.
But depending on your anatomy, and who you fuck, this may not be enough. I remember one party this Spring, where the host saw me angrily scrubing my cock, pubes, etc. in the sink. He asked what happened. And I said there was a major mudslide, and don't these people clean out. He responded "Well, you know, that's a hell of a plunger."
