Female dogs go into heat every 7 months to a year.  You can see that they have started by their swollen vulvar which immediately afterwards they start their period which they'll fight if a male messes with them (sounds like another species,LOL) then after their period a round 4 days they'll start doing odd things like humping other animals or a pillow or whatever now is when she needs a male, now she is prime this will happen for a week or so. Then it's over and done until she's starts swelling again.