Nap time

Here latly it has been a incressing struggle to get Shelby to take a nap. Last week she feel asleep on the floor in the living room. Today it was the same. She fought me for 2 hours to even just lay down. Jon has been home most of the day. All Jon had to do was come in to the house and say "you need to take a nap baby girl" with in 5 min. She was out. How much crap is that? I would like to know when the proper age is to let her not take anymore naps.