I was at about 12 hours when I went solo.
However, the most important thing is that you are ready. You will be making the transition from student to Captain and it's a thrilling step to take.
Think of everything that you want to ask the instructor, and ask it before you go. Cover all the 'what ifs' you can think of. If you encounter an unusual situation in the air, remain calm and think about it.
On my second solo at 300' the door popped open at the top and was flapping in the breeze-so if you are flying a PA28 (or whatever else for that matter) give the closure of the door a thorough testing once the instructor gets out.
Most of all, enjoy it and have a camera to hand for a nice pose in front of the plane when you have finished: you only get one first solo!
Enjoy
Bri