There is a wide range! Ayasha is 17 months and still NO heat cycle! If you search on the forum, you will find all kinds of info and experiences people have had on here with heat cycles. Ayasha's dam was 14 months.
Lilly was about 8 months. She actually started peeing on my bed. I ask my dog trainer about her peeing on the bed and she said that she was going into heat. We got her spayed soon thereafter and the vet said that she was about to go into heat.