Sounds like you're trying to budget for finishing the course.
Each lesson takes as long as it takes Even if you do it perfectly there will be lots of reasons it takes longer or shorter than 1 hr. For instance, weather, traffic, type of aircraft. It's just not something any flying instructor can tell you.
Either way the course takes a minimum of 45 hours - this is easily achievable for motivated people in their early 20s. more if you're lazy and it often takes more once you get past 30. so even if you've finished the whole course by hour 30 (which you wont) you still have 15 hours of flying to do, so how longer each lesson takes is totally IRRELEVANT!
Reading the books on the other hand will save you money, if you don't read the books it will often take you past 45 hours.
Beyond that, paying £2000 in installments to get a big discount sounds like a pile of shate, there are discounts for paying for 10 hours at a time but only ever do so with a credit card, you never know when a school will go bust and it happens lots. All the more important if you're on a budget. Suggest finding another school to be honest, where are you training?