A roof coating may buy you just as much time for much less cash.
I'm not sure if White Kool Seal will work over shingles, but if you look, there will be something out there that will work. Even if you had to recoat once in a while, it would still be cheaper, a better sealed roof, and would only take a couple of hours to go get it and put it on. Apply it heavy with a long handle and a paint roller. Do not step on a roof coating before it has a few days to dry!! The top surface of the coating may seem dry until you step on it and it breaks loose. It takes a few days for most roof coatings to adhere and "set".