15 years appears to be the common period of the Marvel Age of heroes. That is, the FF getting their powers. Regarding Franklin and Valeria Richards, they both spent years away from the 616 Earth and aged at different rates. As far as an accurate metric, they're not really very good. But then again, there really is no official in canon length of time and we seem to be best just going with 15, at least for a while. I do recall 13 was used in the dame unofficial way a few years ago.