DS when younger needed 11.5 nights on average when he dropped the nap at 3.5 yo. After few months he went to 11 h on average and this was pretty much the same until now, at 4.5, with his sleep averaging now 10.75 h. He goes to bed at 9 pm and his gro-clock is set up at 8 am (8.30 in the weekend) and he sometimes wakes before, sometimes after.
What I notice is that his sleep is consistently between 10.5 and 11.5 h, with some odd nights at 10 or at 12, so I really have to look for the average, not for a consistent night length... this is given only by the gro-clock.