Simple answer - 0 minutes!!!
Jack (2 and a half and a bit) hasn't napped for about 4 months now
. It's a mixed blessing - prior to dropping the nap he wasn't settling till 8.30pm and up again at 6am. Now he's dropped it he goes down like a dream at 7.30pm and sometimes sleeps past 8. I have to try and avoid long car journeys after about 3 though otherwise he's climbing the walls well after bedtime.
From the age of 2 I was never letting him have more than 1.5 hours - had to wake him every day and he was grunpy as hell for a good half an hour, but if left he would have slept for 3 hours!
I miss my afternoon rest though