I've heard of anything from 2 hours to 50-odd. There is no right or wrong "number of hours" at which to solo - it is not a competition. Does it matter that you've soloed before or after other people?
If someone says they soloed in 20 hours, does it make you feel better? Or of they say they did it in 5, would that make you feel inferior?
It doesn't matter.
Cheers
Whirls