I don't have a lot... Almost 7 days. Then again, having more than 10 days played on this game would be like stabbing yourself in the heart and not dying.
For goodness sake, can't you just ask in the existing Modern Warfare 2 thread? This forum might as well be called "Modern Warfare" because that's all there is. >.>
On topic, I think I got to 15 days before I got bored. Not too long compared to my time spent on some other games.