I guess it all just depends on who you are - some people laugh for a period of time then quit - like sketch and tickleshow said - and then there's people like myself who just keep laughing and laughing until it starts hurting their throat, lungs, vocal chords, or sides and can't take anymore.
As time rolls on in a tickle session, my laughter actually gets waay louder and more intense, so it's pretty much a question of "When do I feel like my sides are gonna split?" I think the longest I ever went before I had to beg someone to stop was an hour and a half of virtually constant laughter.
I also looked around for a minute to see if I could find some kind of data on the length of time someone can laugh or an official record of the longest laugh... came up with nothin... maybe someone should try to set one?