The rule of thumb to provide a distance on departure is to roll the second aircraft when the previous departure is 1 mile less than the required distance away. Due recognition has to be given to aircraft type and differing performance characteristics (short take off roll and/or fast acceleration) in order to avoid short-changing the required separation.
There is no need to use a stop watch.
If you need a time based minimum, advise tower before accepting a line up clearance with the required time. Asking for another 30 or 60 seconds once on the runway is more often than not, in a mixed mode operation, going to result in a missed approach. In non- mixed mode you'll probably just incur the ire of the controller.