Once my babies settled into the morning/afternoon/early evening nap routine, the next thing that happened was the early evening nap got shorter. Over the course of about a month or so, somewhere between 6-8 months or so, they settled into a morning/afternoon nap routine that was pretty consistent by 8-9 months.
Dropping to one nap a day was a lot trickier. My twins were about 18-20 months when they went to one LONG afternoon nap. My third was about 13-14 months, though he'd have kept napping twice a day had our schedule permitted.
One thing to keep in mind is that most babies don't just drop a nap one day, it's more of a process. They might take that third nap 5 of 7 days one week, the next week it's only 4 days, then 2-3 days, then only rarely, then not at all. Each time we dropped a nap, it was at least a month long process before the new routine was really consistent.
#4 is now almost 8 months old and takes that third nap some days, some days not. We are in much less of a routine than when my older three were babies, mostly because we CAN be (having twins then a singleton 13 months later meant we had to have some sort of routine so I could take care of everyone's needs).
I figure she'll be at two naps a day in a month when school starts (she'll be 9 months), but I have figured in time for a late evening cat nap if she still needs one that also allows for an early enough bedtime so she's ready to start the day in time to get my older ones to school. This fall will be much more time/schedule oriented than ever before, out of necessity, and I'm not sure how that's going to work exactly. We shall see...