It's not a constant reward.  Many factors go into determine reward amount including -

1) How much the developer of the game is spending at the moment
2) How much you have earned recently
3) Promos in effect or not
4) How much traffic is running through the system
5) and more...

It's up to each dev to decide what they are spending at any given time.  Best advice would be to support the dev by clicking on a few ads, or anything else they are trying to do.  $.001 is pretty high considering you can complete it hundreds of times (if I remember correctly).

If a dev determines that you don't help generate revenue (say by clicking on an ad), it's possible they could taper down your individual rewards from a business perspective.  Just a guess there.  I know that if players installed any free apps yesterday from the blue button they were paying $.01 to $.25 USD worth of bitcoin!  (with no limits)

Each game (and developer) will be different at different times.