She will be in for approximately three weeks. For the first week or so she may lose blood, but you may not notice if she is very clean. For the next few days the blood will turn to a straw colour. This is when she will become receptive to any males around, so make sure doors are closed and gates kept locked as she will try to escape to find a mate. They are usually ready to mate for only two or three days, and this can be anytime from about day 7 or 8 up until day 21 or 22, or even longer, but the usual time is 12 - 14 days.
She will probably come into season twice a year, although this varies from bitch to bitch, and some are only once a year. Most will be every six or seven months though.