I agree with Halfbaked - 90 mins is about the instructional limit in the air before fatigue begins to set in and your brain starts to lose it's ability to retain information. One of the most important things to remember is showing up on time for your lesson, well rested, fed and watered and free (as far as is possible) of external stressors.
Get your mind in the cockpit on the journey to the airport and debrief fully after the lesson. Doing this will accelerate the learning curve and make each lesson count.
Turning up tired, stressed, hungover, ill etc is a waste of time and money and will increase your frustration level and slow your overall progress.
Three hours booked lessons per week does not mean you will get 12 hours per month though. If you get half that you will be doing well and at that rate factor on 8 - 10 months minimum to complete the course and be ready for a skills test.
I wish you the best of luck and hope the weather gods are feeling charitable.