Due to work commitments, it took me around 16 months to do my DL course, so thats a similar time frame to what you are predicting. I remember that before I started, I had the same worries as you. I worked out that I could do about 15 hours per week and didnt want to forget everything that I had learned by the time the brush up course came. Using hindsight, I can now see that it turned out just fine. I actually ended up doing between 20-30 hours per week due to the fact that once I got into it, the subjects were quite interesting and flowed really well. The main thing that I would say, is plan your week ahead. I also found it very beneficial to take a few evenings at the end of each month to re read the things covered at the start of the month. The brush up courses are really good and by the time they come around, its amazing how much you still remember. I got 14 first time passes with 92% average in the end. Best of luck.