The length of time it takes to learn to fly a glider is dependent on a number of key factors.
  • Age 
  • Coordination
  • Aptitude
  • Confidence
  • Time between training
  • Previous experience
Typically most students learning to glide will do a combination of both winch launches and aerotows.  To go solo a student must be able to consistently demonstrate to their instructor that they are safe to fly on their own to do this the student will have to have shown:
Good coordination of stick and rudder
Good circuit planning - including landing from out of position locations
Sensible decision making when under pressure
Good judgement of height & distance
It normally takes between 80 - 120 flights to go solo, but occasionally some students learn much faster and others take longer.  If you want to guarantee it's not going to cost too much, then paying for a "fixed price to solo" is a good way to do this, typically this is a more appropriate choice for slightly older students who will take longer to pick up the skills.
If you have previous experience flying powered planes, paragliders, hang gliders or other air/wind based sports you will have a slight edge over other students.