Clean the sponges whenever you notice a drop in flow from the filter. The sponges only need changing when they are litterally falling to pieces, i have a old style Fluval 4 in one of my tanks which hasnt had the sponge changed for 4 years and it still works fine.
I agree with CFC. I just wanted to add that when you do clean the sponges, only clean them enough to get the gunk off and in tank water. You do not want them spotless by any stretch of the imagination as you will disrupt the bacteria.
You are right, if the water stops being able to flow there will be a build up of amonia etc. Just monitor the flow of the water and rinse the sponges a little and it'll be grand.