Any experienced dog breeder will tell you there is no hard and fast rule. The best guide is a male dog, he will let you know if she is ready, and her reaction will also confirm this.
If she flirts with the dog and flicks her tail to one side (she may also do this if you stroke her back) then she may be ready, but if not she will probably tell the dog off in no uncertain terms. The usual time is between day ten and fourteen, but I have had a bitch stand and be mated on day thirty and produced her biggest ever litter. They can be mated as early as day five or six, although unless you are experienced you may not realise when her season actually started.
What breed is it and have you bred a litter before?