There are loads of variables. Your age, how often you train and the type of base you train from will have the biggest effect on the hours in your logbook before solo.
A young student training full time at a small GA airfield could easily solo in 8-10 hours. An older student training only twice a month at a big airfield where he has to fit in with ryanair and easyjet will probably take over 20.
Also, watch RAF figures, they usually don't count any taxi time so for us civvies you can add at least 10 mins to each flight.
So 12 to 20 hours is probably pretty average, but ultimately it takes as long as it takes. Often, a student will be pretty much ready for solo but the weather on the day isn't up to it, so I say lets take a break from the circuit today and cover steep turns. It's another pre-solo hour, but really makes no difference to the total course time.