I try and clean my AR every time I use it, but I've spent a week out in the bush with my AR, kayaking there and back, covered in mud, dirt in the workings and I didn't clean it until I got home. Swam with it through a river and 3 days camping and didn't clean it until I got home. Overnight in -10 weather sitting on ice and snow and didn't worry about cleaning it until I got home... During all those trips I used the rifle and it never once failed to work. HOWEVER... When it got all muddy the first couple shots I had to manually charge the rifle.
Now I know I'm just saying more about reliability, but I didn't have to clean it and it was fine. If you keep it well oiled you don't have to clean it as often, but after long storage clean it and make sure you oil the hell out of it, after heavy use, and if you can after every shoot. Not required for it to still work usually. My gun shoots best when it's not got a lot of oil in it so if you oil the hell out of the gun make sure you clean the hell out of the gun.