This is my limited experience with females: the heat will not start suddenly. First her vulva will swell and she will be marking and become interesting to the males. You have to separate her from males and recently neutered males from that time on until her vulva has returned to the normal size and she is no longer desired by the guys. Most likely she will bleed for several days, week, 10 days, depending on the individual dog. Most will be able to conceive after bleeding stops but this is not always the case so play it safe.
The breeders here can probably give you more detailed info.