Firstly, you'll know when the guy is going to blow because you can feel it working it's way out. I'd say you get maybe a 2 second warning. The main variables are taste, consistency and quantity. Sometimes it actually doesn't taste that bad - maybe comparable to a salty drink but it depends on what the guy has eaten. It can be thick or watery, and sometimes there's so much you have to keep swallowing while other times you can finish it off in one gulp.
The main thing though is to figure out if you'd rather deal with the taste or take your chances on the amount. Your taste buds are stronger at the front of your mouth so if you don't want to risk dealing with the Philly Cheesesteak the dude ate for dinner, swallow as much of his dick as you can. Or if you'd rather prevent yourself from possibly choking, just have the head in your mouth when you know he's ready to explode.
There's a post for you, OnTheRag.
