7 hrs 40 mins but it is not very relevant as weather comes into play. Ie you maybe ready at 7 hrs then hit a bad period of weather. What do you do? sit on your back side going rusty? No you carry on gaining experience and keeping current until the conditions are good for you.
15 hrs is probably a good bench mark but not that relevant. There could be an arguement that you are sent solo when you are ready to do your solo cross countries and attach the lot together.
When I started flying I had a wife and young baby and no money. My ambition was to fly an aircraft on my own and then to drop the lot. 20 odd years later 4000 hrs and an ATP I often wonder what went wrong with that initial ambition
The more time and experience you build up the better as you have to be safer than aiming for a minimum hour to solo record.
Pace