It varies is the answer for sure but I'd say 5ish or 10 minutes is average. It's been longer, it's been (much) faster, but it totally depends on context. Energy levels, general excitement, consumption of booze or other intoxicants, etc.
My unfortunate observation is the longer I am with a partner the more comfortable I slowly become, and the less time I'm able to last.
My first playthrough I sent lae'zel straight to camp, never spoke to her again. At some random point before even act 1 ended, her romance dialogue came up.
My theory is the tadpoles ramp up sex drive to absurd levels, unless you're shadowheart apparently. Illithid orgies must be incredible.