Run the car for about 5-10 minutes to get it warmed up, then drain what you are going to if you are dumping it out the bottom. Getting it warmed up a little bit will allow the oil to drop out pretty quick. Wear gloves (nitrile) if available. The oil will flow fast, so pay attention to what you're doing or you'll dump out too much. If you dump your oil into a clean container, you could possibly put some back if you dump out too much.
As far as how much oil goes through an engine in a period of time, it all depends on what the car is, which engine is in it, what mods are done to the car, how many miles are on the car, what oil is in the car (synthetic/dyno/weight), when the last time the oil was changed, how fast is the engine spinning at any given time, etc, etc, etc. In other words, there are just too many variables to give you a good answer.