sinfulj's example is close to a good solution.
It has a few pitfalls though.
You will want to make sure you are in a 1000 pixels range of your next "iteration".
So have your instance variable "fallDist" which will be equal to 0, 1, and so on. Its default value should be 0 though.
Then your condition should be :
Bullet distance travelled >= [(greater or equal)] to 1000 * (fallDist+1)
And as action, you do indeed add 1 to fallDist and any other of the actions you want to happen on a regular basis every 1000 pixels.