The amount is hard to determine,as it varies ont he level of arousal. The same for question 2 - the time it takes to first show varies on arousal. I can be touching myself for 10 minutes and not have precum yet, or I could be sucking on a dildo for fun and start precumming within minutes. I've seen changes recently, because I'm on supplements. It's been an increase in volume and it also seems less sticky than it used to. Much more watery and less viscous.