Every 7 days, size depends on females. But, the highest we go is medium rats, smaller adults may be on a size between small and medium. Adult males only get smalls.

If breeding, a male may get a small every 5 days, to pack on weight before they go completely off, but normally they do not need this at all, it's only the smaller ones.
Females if they're going to breed, and they are hitting the food hard (REALLY hungry), they get smaller meals more often (5 days, instead of mediums they get smalls). So that they gain more weight that they want, but on a more gradual scale, I don't like them packing on huge amounts, or power-feeding.