As far as I've observed, the length of a trial has no impact on performance in the player, but longer trials do slow down the editor. How severe the slowdown is depends on your browser and your computer. So there is no point at which breaking a trial becomes "a must" for anybody. It depends on how much you're willing to tolerate slow loadtimes and just how slow the loadtimes are given your machine.
Personally, if a single part goes above 3500 frames, I would break it up. Trials above 5000 frames are extremely difficult for me to edit.
[D]isordered speech is not so much injury to the lips that give it forth, as to the disproportion and incoherence of things in themselves, so negligently expressed. ~ Ben Jonson
Current AAO Development Priority: Issue #94: Grayscale Mode