You'll solo when your instructor is confident of your flight skills and your ability to control the airplane and anything that is thrown your way without him/her in the airplane. It can be 15 hours or 30, it doesn't matter. Your frustration is normal, don't let it get to you. Keep practicing and keep studying.

If you fly a 172S/SP, I have lots of material on my website that you can use, - Main Page -.