Here's a quick and dirty explanation.
Personally, this is my take on the whole lamp thing. When a manufacturer says that their lamp is rated for 4,000 hours, what that really means in my mind, is that it's rated for 1,600 strikes. They probably figure that when somebody uses the projector, they use it for about 2 to 3 hours per time. So, if you use the projector for 2 or 3 hours per session, then your bulb will have about 1600 strikes in it (if it's rated at 4,000 hours, and obviously, this is if everything works out perfectly with the bulb).
Now, it's my opinion, that you should basically almost just forget about the hours, and think about the strikes. The bulb only has so many strikes in it's lifespan. So, when I'm using the projector, I plan on using the thing for a good 4 or 5 hour stretch of time, if not more. Using it for only 1 or 2 hours, is kinda a waste of a strike imo. So, lets say on a rainy Sunday afternoon I'm watching football for like 6 hours, but I need to leave the house for a few hours, I'll just leave the damn thing running while I'm gone. No use in turning it off for a couple of hours, and then turning it back on to watch the nite game. I just leave it running, and when I get back home, I just go into the room and watch the nite game. When I know that I'm not going to use the projector for at least 5 hours, then I'll shut it off. Basically, I pretty much never strike the bulb twice in one day. If I'm going to use my projector that day, then I'm going to only use it once, and try to maximize my time with it.
Also, regarding the bulb strike thing... Let's say my bulb has 1,000 strikes in it. I know that if each time I turn it on, I leave it running for a full 24 hour period, that my bulb would theoretically have a lifetime of 24,000 hours. Obviously that is totally ridiculous, and I'm aware of this. So, I know there is a limit to how many hours you can run the bulb off each strike, but I seriously feel like if I use my projector for 8 hours off each strike, it's the equivalent of using a projector twice, for only 2 hours, and shutting it off after the two hours is over. In that scenario, you are only using the projector for 4 hours, but I think the one strike, 8 hour scenario is actually less wear and tear on the bulb.