[Maps-l] Tiles are not cached long enough

Hi. I run a mod_tile/tirex/own-renderer system at home. Since my own-renderer is slow I am interested that the tiles are cached indefinitely. I upped the defaults in mod_tile by a factor of 100, so they should be cached 100 weeks or at least 100 days. But they get re-rendered all over again, and I do not know why. Is there a definite log that tells my why mod_tile thinks they are expired? Or is there a system that marks them as dirty (without my knowledge)? Thanks Kay