Never force anyone to solo! Also ask student if they are ready before you get out of the plane. If they say yes; ask if they are sure. Also don't tell the student in advance, especially as they might invite friends or family to watch. I know this has been done, but another bad idea. Most of us had some degree of anxiety about the first solo, though the instructor must determine if the student is truly prepared. No amount of hours should play a part in the decision to solo. Hours might factor into assessing a students progress and other issues, though flight time alone should never be used as a factor in determining if a student is ready to solo.