just have a geez at it, look at the colour.
Theoretically, as the pod gets older, flow will decrease. At what stage you feel that the decrease in flow is justified to change it, is up to you.
If I was changing a paper filter in a car, I would probably do it every 10 000ks. A pod is supposed to be an upgrade (flows more), but i don't see why you would want to have one on there that's all dirty.
Having said that, I do have an apexi pod, but next change, probably will get something washable like a kn