it can start anytime from 6 months on up. if you can find out when her mom's first heat cycle was and that might help you determine when to be prepared for it. Its kinda like with girls who get their first period. Usually get it about the same age their mom and grandmother first got theirs. some dogs can get it as early as 6 months but i've also heard of dogs who didnt get their first heat cycle until they were a year and half! though the later as i'm told is more rare.