A Bitches first season can be at anytime between 6 months to 2 years, if you have other bitches on the premises they may 'bring in' a youngster as they tend to come in together, but not always. Some will be as regular as every 6 months, others will go much longer some as long as a year. It is easy to miss the first season as these can be very light, but if there are any males in the area they will be fully aware and may well come to visit.
Normally the hormones become active some 10 days earlier and there will be some marked changes she will urinate a lot to let the boys know where she is, the vulva will become enlarged and in some bitches really swollen and sore looking. The discharge varies a lot from bitch to bitch and most bitches keep themselves scrupulously clean. A gentle wipe with a tissue first thing in the morning before she has been out to relieve herself will confirm your suspicions. If you want to breed from her it is very important that you are aware of the first day of the season, to enable you to have a rough idea when the best day for mating is. This normally takes place between the 13 -15 day but bitches can vary a lot so from the first season watch her, the discharge will pale off and the vulva become soft and flabby, she will often stand for you if you rest your hand on her rump, (it helps if you keep a record each season to find a pattern). During this time she should not be taken out on exercise and when she is in the garden keep a watchful eye as she will be more than likely looking for an escape route and the dogs in the area are just as likely to try to get into your garden.
It is best not to mate a bitch until her third season, certainly not under 20 months. - Mastiff Club Code of Ethics