The right time to leave a hook-up is highly dependent on context, so just stay aware. Preferably, ask him if he has a condom before you leave to meet him. Have safe words and build a bond with your partner before exploring such fantasies. Touch his arms a bit and let him know you are comfortable with making physical contact. Do be clear with your expectations for yourself. But if booty call is the name of your game, go forth and multiply your orgasms. The only thing that is guaranteed to work for me is oral.