I think a lot of how you approach dropping the catnap depends on how long your LO sleeps at night and if he/she does make up for sleep at night. So, part of it is trial and error (figuring out the aforementioned things). That said, how long does your LO sleep at night normally?
For us, DS is an 11h night sleeper, and he doesn't make up for lost sleep at all. I ended up doing a 13h day with an 11h night, sometimes a 13.5h day/10.5h night just because an early bedtime for him would usually mean an EW. I kept the catnap until his A times were ~3h long. Then, once that happened, I dropped it. I could safely do an early bedtime of about 20-30 minutes early if need be, and it would throw off his awake time too much.
HTH a bit
Also, the average range of dropping the catnap is 6 to 8 months...so if your LO isn't ready to do it quite yet, there's absolutely no rush