My DD went down to one nap at 10 months. On the flip side, she's 3 and still takes that one nap. DS is down to 2 naps (morning and afternoon) and not sure when he'll drop down to 1. Every kid is different.
I think in general it's around 15-18 months, though I've heard naps get wonky around 10/11 months and they'll start acting like they only need 1 nap, but it's temporary and they go back to two. Every baby is different, though.
Re: Down to one daily nap?