Biggest mistakes I see regarding length of clips are:
1) Ler does the same thing for the entire clip. This makes a 6 min clip seem like 20 mins and the viewer will lose interest quickly.
2) Ler only spends part of the clip actually tickling the lee. Lots of "spontaneous" breaks are given to the lee as the lers banter with each other or whatever. That's just poor form, and is clearly a way to use up time while not actually tickling the lee.
3) Clip ends abruptly during an especially hot part of the clip. I mean, really? If you've just discovered that the lee has ticklish nipples, and the lee is going crazy over how ticklish their nipples are, this is NOT the place to end the clip. LOL. That discovery should've been in the middle of the clip anyway.