lulu,
For my lo, if she does 2 naps and one of them is a decent one, BT has to be at least 8:30 to push her WU after 6 ( but that's is my lo who only does 9-10hrs nite, if yr lo can sleep longer during the nite, u mayb able to put him down earlier, and with a nap so late, it is very likely that she'll get a long NW too), so if I were u, I'll keep it at 30mins. I know, it is disheartening to wake yr baby when he is sleeping sweetly, but I'll rather wake him at 5:30pm than he wakes himself at 3am....
For us, yesterday was a one nap day, she had a 1hr15 nap in the am, but then she refused the pm nap and we were out so AP was impossible. Ended up with a 7hrs last A and BT at 7:30 again. Needless to say, early wake up for us at 5:30 but she STTN for 10rs (with a little cried out for 2 seconds) so I shouldn't be complaining.
Today is definitely going to be a 2 naps day, I put her down at 4hr A for 40mins, aiming a long pm nap at 3hrs A.....hope it works.