My quickest would have been a number of hours, but I had known about him for 10yrs. We weren't friends, he was a friend of a friend of my mum. Then one day I happened to be hanging out with them and bam.
I wasn't actually interested in him - he was just eye candy with terrible sex! I was far more aroused by just looking and smelling him.
If I'm seriously interested, I'll wait a few weeks. I'm not giving it up too quick if I expect him to stick around.