R.D.
If you haven't already, develop some sort of pre-nap routine, so she'll start to recognize the actions as naptime. Read a book, rock in the rocker, quiet time in the darkened bedroom. Just some ideas. Both my kids went through this stage of 30 minute napping, and I don't remember how long it lasted, but like the other poster stated, consistency is helpful. Once she gets into a routine and starts to recognize the cues that this is nap time, she'll get into long sleep patterns.