I always wait until they have at least 25 hours. Regardless if they are ready or not I would not want to have the conversation with the FAA about why they went solo at 10 or 15 hours. You can always do the night landings, short field landing‘s, and soft field landing‘s as well as some airport hopping if you need to build up to 25 hours. These things need to be done shortly after solo so you might as well just do them before to build up more experience and increase the chances of them having a more successful solo.