When your toilet is working properly, it should only run for a minute or two after you flush it. Once this time is up, the tank at the back will be full of water, and there should be no reason for your toilet to run again until you flush it again. If your toilet is running for more than a few minutes after you flush — or if it is starting to run when you have not recently flushed it — you have a leak.
Where is the leak?
There are two main possible explanations for the leak. Water could be leaking from the tank into the bowl around the edge of the flapper, which is the plastic or rubber piece that fits over the opening of the pipe leading to the bowl. Or, there could be an actual crack in this pipe, causing water to slowly leak into the bowl from the tank. Since the tank is slowly emptying, the toilet needs to run and fill the tank periodically.
How can you fix the leak?
Unless you can actually see a crack in the pipe, you can start by replacing the flapper and hoping that this fixes the problem. Flappers only cost a few dollars at the hardware store, and they only take a few minutes to replace — so you don't have a lot to lose.
Turn the water supply off to your toilet by closing the main valve at its base. Unhook the flapper from the pipe, and disconnect the chain that connects it to the flush arm. Purchase a new one (flappers come in one standard size), and hook it in place.
Once the new flapper is in place, flush your toilet and see what happens. If it fills normally after you flush and does not start running again, you've solved the problem. If the toilet still runs abnormally after you replace the flapper, then there's probably a crack in the pipe itself. Contact your plumber to have this issue addressed.
How will your plumber repair a crack?
Depending on the severity of the crack in your toilet pipe, your plumber may be able to repair it with some epoxy. The epoxy may take a few hours to harden, which means you'll have to do without your toilet for a few hours. If the crack is severe or located in an area that makes it hard to reach, your plumber may recommend that you replace your toilet rather than having the crack filled.