[thastygliax]

As Andrew Hackard said, Pathfinder has no official rules for play after 20th level. There are some suggestions on pages 406-407 of the Core Rulebook for how to handle higher-level play (including XP progression), but that section isn't meant to support long-term play at those levels.

Paizo eventually chose to develop their "mythic" rules rather than advancing characters past 20th level. Those rules provide much of the same experience as D&D's epic levels did--characters who can do things mere mortals could never dream of--but those elements are integrated into play at lower levels. The exact level at which you introduce mythic tiers is entirely up to the GM. It can be as low as 1st level or as late as the 20th (or just before), but most of Mythic Adventures--including the sample adventure--seems to assume most games will introduce mythic elements in the middle levels (roughly 7th and up, IIRC?).