I love teleporters. They're convenient on nearly every map, whether you're attacking or defending. And secret teleporters are the best, because as long as they're secret, they're safe.
But, a tell-tale sign that there is a teleporter somewhere nearby is the lingering particle effect that clings to a class aftere emerging from a teleporter, and the last thing I want to do to my good ol' Engineering buddy is to reveal the location of his hidden (rebel) base.
So to sum up, how long after exiting a teleporter is the particle effect on a character model visible to enemies?