It took me three summers (I dont dive in the winter in Cananda) to complete my DM. The deal with my instructor was that the course sost me 0$ but I had to help out and assist plus the rest of training. I did approximately 15 classes and 10 Cert as an assistant before I got my DM. I learned most of it through experience what was actually taught in class was intereasting but nothing really new. I'm doing my instrcutor nw and that will take me about 5 months after DM. The books are nice but there nothing like experience,my thoght is a DM or Inst should know books and theory but the rest is what really will make the difference in your training. (because seriously if your from NAUI,PADI,PDIC or another agency the books are all the same or mostl with the expception that a PADI dive should be be like this...yada yada yada, a NAUI dive should be like this ...yada yada yada , a PDIC dive should be like this Yada yada yada...) oh and the PADI,NAUI or PDIC book you should buy.
My suggestion is get the best experience from different instrucotrs possible and the length of time is irrelevant.