Heat cycle

Question
I have German Shepherd and she had her first heat at 9 months old, she had her second heat 6 months later at 15 months old,it has now been almost 7 months since her last heat. When should she go heat again. I thought it would be 6 months from the previous heat. Thank You Sue

Answer
6 months is typical, but being biological, it can vary.  

Your question suggest that you haven't done the serious work responsible breeding requires.  Why don't you go ahead and have her spayed?  See http://www.woodhavenlabs.com/articles.html#breeding and http://www.avma.org/animal_health/brochures/spay-neuter/spay-neuter_brochure.asp