I'd look at her overall rate of weight gain. My son was a big kid, he kept gaining steadily until he was about 6yrs. My dd however, has been tall and thin since birth, and pretty much quit gaining around age 3. Our seat was only 33# RF and I turned her right around age 3 and now at age 4.5, she still bounces between 34 and 35#, in fact at one point this summer, she was back down to 33#! You never know with some kids.
Anyway, I'd start weighing her every month or two and see how it goes. Try not to be obsessive about it but write down the weights and look for a pattern of weight gain to give you some idea of what the future might hold. You could still have a very long time to RF if a 35# RFing seat.