I'm surprised at the number of people here using the timer feature on their drier, and are not using the moisture sensor, which would have an unknown length of time.
I can't speak for others, but when I discuss this there's a difference between "timed dry" and what I'm referring to as the timer. I use the automatic dry modes and I agree, the time can vary (making it difficult to know whether that initial time the dryer gives you is correct). My washing machine also has a timer that starts when you start a load, but that time changes depending on the load size.