I have 13 month old twins. They usually nap for 1 1/2 hours in the morning, and 1 1/4 to 1 1/2 hours in the afternoon. Sometimes they shorten the afternoon nap, particularily if I let then sleep a bit longer in the morning.
My pediatrician said that when they start napping for less than 1 hour, it is time to consolidaten the naps. She said they need an hour to reach REM and resurface.
I do find sometimes (for the last few months) for a few days running one or the other seems to boycot the afternoon nap, fussing or playing for a while before settling, or waking up way too early, but I just carry on and it seems to go back to normal. I hope to carry on with two naps for as long as possible, but I think most kids transition by about 18 months.
Sounds like your little one still has two great naps a day. Congrats.