A: "Look at the cool jetpack I got!"
B: "Thanks, but how long can you stay up there?"
A: "I don't know that! Whoo-Aaaaaahhhhh!!!!!!!!!!"
If someone puts this in their mod, I'll probably spectate an ATCS match or two just to watch noobs drop out of the sky like dead birds.
If you are going to put a timer on it, make it slowly on stamina. When it starts to run out make the jetpack sputter and fall, so that every alien in earshot knows to go outside and catch him like firemen with safety nets.