There is no hard limit, only a probable one. P1 factoring finds factors p of a number if all of the factors of p1 are less than a certain limit most people call B1. Using an extension, p1 factoring finds all factors p if all but one factor of p1 is less than B1, and the remaining factor is less than B2. It is conceivable that this method could find an arbitrarily large factor, it is just highly unlikely. Almost all factors found by this method will be less than 35 or 40 digits.
