20 hours.
Because SFAR 73 in the US says you cant solo in less.
(student pilot in an R22)
I must say although its perfectly possible to send a student solo with 7 hours, it doesn't strike me as particularly safe. better to err on the side of caution. Of course, time to solo depends as much on the instructors experience as the students. I was ****ting myself the first time i sent someone solo!