Another "rule of thumb" which I believe I can attribute to Ansel Adams, concerns watching for the appearance of mid-tones during print development. His rule was to time the appearance, then triple this time for optimum development.
For example, Polymax Fine Art in fresh Dektol at 68 degrees begins to show mid-tones at 35 sec. Development would therefore be complete at 105 seconds.
If your paper/developer/temperature combination gives results at 42 sec., then optimum development would be 126 sec., etc.
Note that these are all close to the 2 minutes that most of us use.
Cooking a little longer to taste would be OK, but undercooking would rob you of all the tones you're paying for.
BTW, if I can't remember how old or worn my developer is, I time the first sheet, and if it's over 45 sec., out it goes. During printing sagas I will time the developer occasionally to make certain I have not overused the developer.