We are usually allowed to leave after the first half hour, not sure about any other limits such as not being able to leave in the last X minutes since I usually leave before then. I leave when I've checked the paper over once, usually to find any totally illegible handwriting and correct it. (as I found myself doing during English on Friday

)
Don't see the point in sitting staring at a paper you've finished for ages, mind starts playing tricks on you and you start to see errors that aren't actually errors