(1) If the girl is really into the guy and really attuned to what's going on, listening for how pleased he is with what she's doing, she gets turned on when he does. (If he cares about how she's doing, maybe he offers to turn her on with a bit of fondling as well.) They're just...ON.
(2) If the girl's nervous about this, chances are she's not as into it as she might be if she was confident that she could please her guy. She may not c*m. (Here, a bit of verbal encouragement, maybe rubbing her shoulders or telling her that what she's doing feels good -- or suggesting something she might try -- would be helpful.)
(3) If the guy insists on being rough, again, the girl may not feel inclined to c*m. Try not to grab her head and get rough. I understand you're not thinking clearly because you're about to c*m, but don't break the gal's neck or make her puke. That's just no way to encourage an encore...
How long does it take? Depending on the girl, seconds ... or years. (Yes, really.)