Julie - my ds was around 7 months before he dropped the last catnap of the day. And even now (he's 8 months), we still sometimes have "quiet time" in his room, which is just me rocking with him for 10 or 15 minutes. Sometimes he doses off, sometimes not. But it's a quiet time for him to calm down. Otherwise he would not make it from 3 until bedtime at 7!
I starting dropping the catnap when I noticed ds would just crawl all around his crib at that time, and would never drop off into sleep. I probably let him do this for one or two weeks before I quit putting him down altogether at that time.
Good luck!