My dog is 4 and she has not been spayed.
Her first season was at 12 months and every 6 months after that.
You will most definitely know when she is in season!! For me it's always other dogs who let me know in the first place, as both girls and boys start sniffing her a lot! Then come the physical signs, which you can't really miss, as was said previously the lady bits will become very large and your girl will start licking herself a lot.
I don't have any carpets to worry about in my house, there is spotting although my dog keeps herself clean.
Her behaviour also changes a lot, she becomes very needy and affectionate and clingy, and there is one week, usually from day 10, where she becomes very agitated, doesn't sleep much, doesn't eat much, and humps cushions and legs (yes girls do it too!) - my kids call it 'humping season'.
I don't take her to high traffic areas for the 3 weeks she is in season, we stick to quiet woods!
I have had a litter from my dog and I think will do one more, then I will seriously consider spaying her to the risks of pyometra and mammary cancer.
I am thinking of getting another female cocker spaniel, if I have her spayed it will be definitely when she has had at least one season and is fully grown and mature.
How are you planning to have your dog groomed? Because clipping will change her coat anyway...