For us it was a very slow transition. It was probably around the 10/11 month mark when it started. We'd have a few days where we'd only get one nap in (in the morning) and then flip back to two for a couple of weeks, and back again, and again. But by the time he got to 18 months we had settled into consistently having one sleep a day.

Funny, we're just starting the same thing all over again with going to no sleep. This time though I'm happy for it to drag out for as long as it takes, longer even!