humm... I think of it like a batter working a pitcher in baseball when you get the right pitch swing at it.
If you take 3 swings and miss cleanly on all three you're out.
How long in real terms?
Let mutual desire and the situation dictate but don't start out swinging at bad pitches, and for goodness sakes don't keep the bat on your shoulders and get caught looking at something you at which you should have swung.
capitio?
In short express your desire clearly, either physicaly or verbaly. As different people respond differently in word and deed and there is often a disconnect.
Be strong but not agressive. Use your head.

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Sex is a tricky thing best left to professionals.