^I saw one of my friends get serotonin syndrome once. It was not pretty. He described it as one of the worst feelings he's ever had, from an onlooker's perspective he was laying on the ground groaning like he was in extreme pain and vomiting. I'm not sure how long it lasted because I was on DXM at the time, but my guess would be 30 minutes or so, but it really is a wild guess.
Serotonin syndrome can kill you though, it's not something to fuck around with. I'm not sure how long you need to wait for 5-htp, my friend that got serotonin syndrome got it because he was on a pharmaceutical SSRI, then took a moderately low dose of DXM (I think he took like 300-400 mg) along with a lot of tobacco. Serotonin syndrome an be pretty horrible and can be fatal, so I'd wait at least 24 hours, just to be safe.