Time between plays is not fixed, but all plays that fall within a certain degree of results are going to come up about the same time. If you see a longer play, you should see more time. That's not to say that the time to run a play cannot be adjusted. If you see particular examples where you think the time should be longer or shorter, let me know. There is a minimum time on the plays and it might be 4 seconds and if so, it might just be that I need to adjust that to 3 seconds if you think a 1-yard or TFL play should be 3 seconds. Keep in mind there is time to start the play running, the actual rushing, and then a small fraction to whistle the play dead, so it's not just how long it takes a player to run 1 yard.
It is not calculating a random time for each play because that saves a little performance in getting random numbers, but I could add that back in.