The most common is with a Fluidmaster 400 . Use the clip that comes with it and do not just stick it into the overflow tube or it will go on and off every so often by itself. If that isn't it, take some food coloring and place it in your tank. Wait a few hours and if some of the color has gone into the bowl then you probably need a new flapper. That depends on a number of variables and which parts.
Water quality is one of the major factors.
If your water source is heavily chlorinated then many parts won't last long. Also the quality of the parts matters. If you use a bowl cleaner some flappers won't even last one year. If I felt it was the right thing to do, I was for it regardless of the possible outcome. We are part of The PlumbingSupply Group.