As a general guide anytime after six months, although they usually come in season about seven or eight months. Bigger breeds tend to be a bit later, usually about nine or ten months, although some never come in until they are anything up to two years or more. Seasons can occur regularly even six months, but again there is no set rule, and they can be six monthly, seven or eight months apart or even yearly (a lot of greyhound bitches are yearly). It is a case of wait and see and make a note of the date you first notice any signs.
Usually slight swelling of the vulva, followed by slight bleeding which you may or may not notice depending on how clean she is. Some have clear or silent seasons, so you never know they have been in (although any local males will soon realise). Word of advice, keep her well contained when in season, as she will do all she can to get out to find a mate when she is at the crucial time to be mated (usually around day 10 - 12 but again this can vary greatly from bitch to bitch and even from season to season). If you are going to have her spayed then halfway between seasons or roughly three months after she has been in is the best time when her bits have returned to normal.