Question about Hair Color

I have been getting my hair colored at a salon. My hairdresser has been using Goldwell Demipermanent color. The problem is that while it covers most of the gray it does not cover all of it and it comes out so quick. I get it done every 6 weeks, but by that time the gray is really bothering me. She had said that sitting under heat after applying the color should make it last longer and that we could try that next and if that didn't work than to go to permanent color. I have very long hair and don't want to damage it to much. My question is will heat really help the demipermanent color to cover better or am I better off getting the permanent color. I am really getting sick of having the gray show after washing my hair a few times. Also how often do most people color their hair.

 

Thanks