Projectors won't last longer or shorter based on usage. Well, generally speaking. 4-8 hours a day of use is certainly reasonable and would be considered acceptable. Actual projector life will depend far more on manufacturer/make/model than your usage. If it is going to break in 5,000 hours then it will break in 5,000 hours no matter how much or little you use it, not because you use it 5 hours a day instead of 2 hours a day.
I always tell people that front projection has a certain cost associated with it which people just have to accept. They often aren't as robust as flat panels and they have far greater heat to deal with in a small area which can significantly reduce their life compared to a flat panel. So, they will fail sooner more often than flat panels will.
Still, I would expect you to get 10,000 hours or so out of most projectors without issue. DLPs have color wheel issues, LCDs have polarizer issues, and they ALL have electronics issues. So, just keep it in a well ventilated area, and enjoy it without thinking much more about it.
Unplugging it at night, after a full system cool down, is probably a good thing IMO.