If you DO spay, you should wait until your dog is at least a year old. However, she may have a heat period before 1 year. A female can come in season as early as 7 months old.
When she is in season, you need to be EXTREMELY careful to keep her from being bred. Male dogs will do ANYTHING to get to a female in heat! They will dig under fences, destroy their crates, and even chew/claw their way through doors. I've even heard of litters being conceived through a chain link fence!
So, do NOT let her outside unsupervised when she's in season. Even experienced breeders get caught now and then - have a friend who's husband turned his back to get a cup of coffee while he was feeding their two dogs, and OOOPS!! One of my dogs was conceived when her mother managed to open her kennel, and climb an 8 foot fence to get herself in with the stud. Fortunately, it was a mating the breeder was planning to make, just NOT on that heat cycle!!
She will be receptive to mating (and getting pregnant) from about day 10 on. (Average is about day 12.) That's about the time her discharge will change from blood-red to pinkish. The fertile period lasts about 5 days, though I've heard of litters being conceived as late as day 18.
No two dogs are the same. The average heat period is 21 days, but last time my bitch was in season, she bled for a full 30 days! I was starting to think something must be wrong, when the bleeding FINALLY stopped!
I know you are also thinking of breeding her, so here's some things to seriously think about:
A German shepherd can have litters as large as 10 to 13 puppies. Are you prepared to help mom raise that many pups until they are ready to go to their new homes at 8 weeks? Can you find homes - GOOD homes! - for that many pups? Do you know what to do for a pup that's sick, or too weak to nurse? How to get a pup to take that all-important first breath, if it doesn't start breathing right away?
Have you considered the cost of raising the puppies? They will need to be wormed and have their first shots before being adopted. Also, if mom has a difficult birth, and needs the help of a vet, maybe even a C-section, have you got the money to pay for that?
These are just a few of the things a breeder has to be prepared for. It's not a job for the faint-of-heart!
i know some people on this board are being tough on you. That's because we care very, very much about dogs, and want to make sure anyone who breeds, breeds pups that are going to go to good homes, and not wind up in shelters, or be euthanized.
We've all been where you are at one time - crazy in love with our first German shepherd, and thinking what a wonderful dog he/she is. Then, as we learn more and more about the breed, we find out, well, maybe our dog wasn't all that great (but we still love it to pieces, and will never forget it...) I've had German shepherds since the 1980's and have yet to breed a litter, because I realized either the time just wasn't right (money, other demands on my time and rules about how many dogs I was allowed on my property) or the dog I picked wasn't as good as I'd hoped. The one with the 5-star pedigree had a dysplastic hip, and the next one had a crappy, fearful temperament, etc. etc.
(The experience I have with breeding comes from working for a breeder, and helping to whelp and raise multiple litters.)
So, please, just relax, enjoy your dog, and do your best to learn all you can. The people on this board DO want to help, but you've got to be willing to listen, and not get defensive.