My whirlpool dryer does not stop when in the auto dry cycle and takes a long time to dry a load on the timed cycle, I clean the exhaust and trouble shoot to the best of my ability.
Can anyone help me, I believe is the timer but I'm not sure

Did you clean out under the drum where the blower is? Normally there is a lot of lint build up in that area. This will cause the symptom you are describing.

Response from caito:

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Yes I already cleaned under the drum and every where else possible. Do you have any other suggestions? Thanks, Caito

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Check to make sure the door is closing good and making a good tight seal. It is important that the only avenue for air to enter the drum be through the element.

Finally, if the dryer is free of lint, and is all sealed up, then it can be only one thing, the element, or something that controls the element.

Does the dryer heat at all? If so, then the element is probobly good, it could be the thermostat, the high limit, or possibly the thermistor.
As you said this symptom also occurs when in a regular timed cycle, Im leaning towards the high limit or the thermostat.