Well.... I personally, would say that it has more to do with the leafspan than the number of leaves. And both of those have some of the trickier (read - likes to have more than one growth to bloom) parents.
I've had phils bloom on one mature growth and a start, and other varieties that prefered to first bloom on 4 or 5. roths are notorious for liking more than one growth to be mature between blooming. I'd say size wise with roth, 24", generally. St. Swithin, well, mine was about 24" with a 3-4" new start when it spiked last year.
Not sure that helps, so, FWIW, both my roths (unbloomed with one mature growth) have 4 pairs of leaves. My mature St. Swithin has 3 leaf pairs, and a new start that is 4-5" LS. I have a phil. and a roeb. that both are taking their time blooming and have at least 4 mature growths each (probably need more light, but I've been working on the roths, lately)!