I generally try to wait until the ball is in the circle and all play has stopped before I grant a time request. I may call time on my own if the runner is in an awkward position and can't get to her feet without releasing contact of the base - AND all other play has stopped.
I'll tell the players/coaches that if I call time and the defense throws the ball away, you've lost an opportunity to advance. But I'm much more selfish than that, since what I really mean is if I call time, it's going to be much tougher to get an out.
