You cant trap the individual key presses of the keyboard in forms. This is one of its biggest failings IMO. The nearest you will get is an ON-LOCK trigger, and that will only fire once per record in base table items. You will need to reset the timer using a number of different triggers, such as WHEN-NEW-BLOCK-INSTANCE, WHEN-NEW-ITEM-INSTANCE, WHEN-NEW-RECORD-INSTANCE for example.
However, if you have a large text item that requires the user to enter a lot of data your timer could time out before they have finished entering their data!