A female dog (bitch) will come on heat twice a year usually.
Usually these cycles with begin when the bitch is about six months old, although this will vary a little between breeds and individual dogs.
You should not get worried if your female puppy has not cycled at six months, but if she still has not come onto heat once she is a year old it would be advisable to have a vet check her out.
It is generally not wise to allow your bitch to get pregnant in the first cycle as very young dogs often suffer problems resulting from an early pregnancy, and the survival rate for puppies born to very young bitches can also be very low.
Watch for the signs of heat cycling. These include a swollen vulva and possible bleeding from the vagina. You may also notice an increase in urination. There will also be an interest toward and from male dogs.
The heat cycle normally will last for about three weeks, although this will again depend on the dog. Between the 11th and 15th days of the cycle are generally considered to be the most fertile, although breeders will test progesterone levels to identify the optimum fertility of their bitches.
Female dogs can only get pregnant during these heat cycles, and will not usually allow male dogs to copulate when they are not cycling.
Be aware when out walking with your bitch during heat cycles that she will attract attention from male dogs, and they may respond quite aggressively in pursuit of her. If she is not kept on a leash you will have trouble keeping her away from other dogs also. The other thing to be aware of when walking your bitch on heat is that she will be leaving a scent trail around the neighborhood alerting all male dogs to her condition and ultimately her whereabouts. Your house may be besieged by roaming male dogs!
If you are not planning on breeding from your bitch it would be wise to talk to your vet about having her spayed. This will be less stressful for you, and will prevent the advent of unwanted additions to your family.