Usually anywhere from 6-24 months. Larger breeds tend to be on the older side for their first heats. Essy was 12 months for her first heat.
You may notice her licking herself more frequently. Her vulva will begin to enlarge (very noticeably larger). She will begin bleeding which will last about 3 weeks. You will need to be ever vigilant during this time because she will attract every male from miles around.