i had a tens machine during my pregnancy, it's the only pain relief i used. i never used the tens machine longer than 30 minutes on a lower setting or 15 -10 minutes on a higher pulsing setting. there really isn't a maximum time limit, it really depends on you and how good it makes you feel. after a while it stops working (the pain relief not the machine) so i just turned it off but kept attached then i would turn it on again in an hours time.
try different variations each day so that the pain doesn't get used to a wave pattern, this way it stays effective for longer.