Just on a break @ work, and thought I would reply to you...
I noticed around this time..or maybe a tad bit later that my son dropped his cat nap. If they need it though...and thats up to your LO of course lol.....then there is no set time to have to keep them up until, or after the cat nap.
My son used to catnap till 5, then in bed @7..sleeping. So, once I saw a change in the cat nap..getting later etc...I took it as a sign that hedidnt need the cat nap as much..and I began to try to extend his A time between last nap and bed time.
HTH