I have a 2 1/2 yr old manufacture home and from almost day one both toilet have been running water into the bowl all the time. We have changed the flappers in both twice. The water may stop running for a short time, but always starts again. Besides increasing our water bill, the master bath runs so much that the tank refills often and wakes me up during the night.

Check the he flapper seal. You need to inspect the sealing surface for any burrs, pits or cracks. Most of the time they can be removed with 400 grit wet sandpaper. Flapper alignment can also be an issue.

Turn off the toilet water. Flush the toilet. Use a wet vac to remove the residual water. Feel and inspect the sealing surface.

There are a lot of other issues it can be too: Toilet Valve Repair - How to for the Do It Yourselfer

Sounds like your flapper linkage needs adjusting.
With the flapper seated the linkage wants to have 1/4" of play. Too much play and the flapper doesn't get pulled back enough giving you a short flush. Too tight and it lets water seep past the flapper. I adjust the chain by the link and if it needs fine tuning, I bend the flush lever rod a bit untill I have the desired play in the linkage. One more thing about a flapper. you will see where the old flapper hooks onto the base of the white overflow tube. On a older type with no hooks the flapper has a ring that slips down over the overflow tube to the seat. If your tank has hooks, take a sharp knife or scissors and cut the neoprene ring off on the marks provided and hook the flapper on the hooks. Leaving the ring on will interfere with the flush. The water level in your tank should be 3/8 to 1/2" below the top of the overflow tube when the tank's filled. Good luck, Tom