The UK microlight syllabus requires a minimum of 25 hours flight training or 15 hours for the restricted licence. This is a minimum and in my experience, it takes most people longer to complete the licence, particularly for fixed wing microlights.
I have seen a couple of students complete in 25 hours, but they were both very young and training intensively.
Generally, age and lesson frequency will slow you down.
Some statistics recently published by the BMAA (British Microlight Aircraft Association) showed that the average time taken is nearly two years and 57 hours of flight training.
I have heard some instructors say "add ten to your age" to calculate the amount of time it will take you. Whilst this seems broadly true, there are other factors that affect it.
Why haven't I soloed yet?
Students often become fixated on flying solo and occasionally they feel ready before their instructor considers them ready. This doesn't mean that your instructor is ripping you off. It is a huge responsibility to send someone solo and we are mainly looking for consistency during landings and emergency procedures. Of course, they could be milking you...but I don't know of any instructors who do that (though I'm sure they exist). If in doubt, visit another flying school and get a second opinion.
Be patient for your first solo.
Remember that today's microlights are quite far removed from their distant cousins of the 80s. A modern EV-97 Eurostar or Ikarus C42, or even a Quik R are very capable machines. They are often more capable than your average Cessna 152 or Piper Tomahawk from your local PPL flying school.
It is tough to complete in 25 hours - don't be dissapointed if you don't.
For microlight practice exams, see here.