Since we're talking motivation here I can compare my approach with that of a friend of mine. He lasted about 4 months, I'm still in it, now for 18 months.
First difference between us was the why. He wanted to gain strength and especially build muscles as fast as ever possible. I just wanted to workout three times a week every week. No direct further goals. He found out that it's easier said than done. Without watching your diet, add supplemental proteins etc. Those muscles just won't grow, not to talk about growing fast, and that killed his motivation quite fast. I, just wanting to go, found out while I don't get larger I do feel a lot better. I sleep better, I'm more alert all the time, have more energy, short: I feel younger.
He started in June. From day 1 it was hard for him to motivate himself when it was so nice to sit outside, have a drink with friends and family. I started in October, when it gets wet and cold outside. When spring came I was already so deeply in my routine that motivation is no problem anymore.
After about a year I noticed my motivation left me a bit. Luckily I found jefit and set myself the goal to get this message about having set a new 1RM record every time I go. It became a bit addictive even.
So after 18 months I'm still going. I didn't get a lot more muscular but that wasn't my goal anyway. But I did loose some weight and I will go on.