I've been sitting at the same completion percentage and trophy count for... about 20 months now, with exception to the 1000 car trophy. Everything done except four A-Spec enduros, and a silver on the final Vettel challenge. Level 40/40, and had gold licenses within 24 hours of the release.
I haven't played it in any serious capacity for months; I turned it on this morning to take some quick shots for a Photomode competition, and prior to that, I had it on in the last week of October to take some of the Citroen GT; it was needed for a class assignment, and I couldn't find the necessary angle with real life pictures. Prior, I can't really remember the last time I raced; I did the Academy stuff in the summer (so technically, not GT5

), but I want to say I did a handful of Seasonals then for some unique Level tickets.
The first year was much different, of course. Played a massive amount in 2010, had almost daily Nurburgring Open Track days with a few GTP buddies, took tons of photos, and did a lot of research for the paint chip database. It was fun, but too many things grated on me compared to the previous iterations - the menu design, the load times, the standard cars - that I just couldn't stay as interested as I did with the older games. Nowadays, FM4 gets more play time, but GT still holds some appeal, with a few cars that aren't available in any other game (ahh, my beautiful '70 Galant), the weather, the 'Ring, and far and away the best photomode in the business.
My overall gaming time has taken a drastic cut since release day (I was unemployed then, and now balance 30 hours of work a week with 25 of school), so to be fair, all of my games have taken a pretty big cut since September. It took me almost a year to beat Uncharted 2!