When will you stop thinking about your ex ?

I was in a two year relationship and it ended in May of 2012 . We both moved on and I'm happily in a new relationship (4 1/2 months strong) Every now and then I think of the fact I wasted 2 years of my life on someone and if they even care about what we had .. I even still tell my new girlfriend stories of how miserable I was in the relationship .. When I was 2 months into my new relationship my ex was going through my Instagram liking pictures and we both didn't even have each other as a friend which means she was stalking me still .. I then set my page to private shortly afterwards ... The main question is . When will my ex stop running through my mind ?! & Why does she probably still run through my mind ?