There is no hard and fast rule when a bitch will have her first season.
Some bitches will start at 6 months, whilst I have know another bitch to have her first season at 20 months. But there are usually tell tale signs of her vulva swelling and if you go near to a male dog, they are very good at telling you. On the other hand some male dogs can take an interest even if a bitch is not coming on heat. Just keep an eye on her and I am sure you will tell. Usually a bitch has a season twice a year, but again no hard and fast rule, it could be every 8 to 9 months. Sorry if this confuses you
I haven't much to add to Megaloo's reply, apart from to say you may also notice that your bitches recall becomes poor about a month or too before she comes into season. My experiences include 4 bitches, three GDBA bitches who all waited until a few days before they were a year old to come into season for the first time and now my own Lab who is currently in season at 9 months.
I'll think you'll know when Ruby is planning something, but it's worth remembering it usually happens sometime like just before you go on holiday or just when you've had that new beige carpet laid or, like one of our bitches on Christmas Day! :lol:
A 'must' read for anyone who has a bitch is The Book of the Bitch by J Evans and K White. A really brilliant reference book for owners of bitches. I saw it in WH Smith the other day, and would guess you can get it on amazon.