I've met people for the sole purpose of having sex so that would be pretty immediate on actual meeting. Other than that... maybe 20 minutes? In college my roommate had a female friend over. He kept making jokes about how we'd look good together and we should have sex. When he left to go to do some laundry I just asked her 'Do you want to?' She said sure, came over and pulled her pants and underwear down. Had a quickie right then.
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