I wonder that myself. My exABF and I also broke up 3 months ago (on New Years Day) and I still miss him. Sometimes I wonder why I miss him because it's not like we had the perfect relationship. But I also miss all the things that went along with him - his family, his kids, I used to work for him p/t so I miss my co-workers and the customers. I think I sometimes get caught up in all the other things that went along with our relationship (the good things) that I sometimes forget about all the bad things that were there as well. I just found out that his roommate is going to buy his house from him and that really affected me for a couple of days - I guess that I always had this hope in the back of my mind that someday we would get back together and live there happily ever after. But the more I thought about it there really is no happily ever after - with or without the house. My ex did the same thing as yours he told me he barely drinks anymore and he doesn't do drugs, etc etc. However since I live in a small town I run into his friends a lot and I know that he is still drinking a lot and nothing has really changed except for the fact that I am no longer waiting at home for him or wondering where he is or why he chooses beer over me. So I guess there has been a change, a big change, but it is in me, not in him.
Well, I guess I got a little long winded there - sorry about that - I'm not sure how long it takes to get over someone - I just think you have to stay positive and strong and I think (and hope) that the sad feelings will get less and less. Good luck.