The heater will stay on until the operating thermostat reaches 140 degrees. If the venting system is restricted it will cause the heating element to shut off before reaching operating temperatures. If any of the drum seals or door seal is not making a good seal it can cause the heating element to cycle off on the high limit thermostat instead of cycling off by the operating thermostat. Check the lint filter too, it may appear clean but could have a clear coating of fabric softener residue which will restrict air flow. Take the filter out and wash it with warm water and soap using a soft bristle brush.
I added an image below with the theory of thermostat and timer operation.