This is my first experience with an evaporative cooler because we just moved into our house. I have no idea how long these pads have been it he cooler. We never noticed any smells come from the cooler and the cooler seemed pretty cool to us. How do I know when the pads should be replaced? My understanding is that these cardboard type ones last longer than the fiber based ones.
You're going to be up there at least twice a year, to put on and remove the jacket, so check the state of the pads each time.
You won't need to replace them every year, but treat them like any filter - if it looks dirty or clogged, it probably needs replacing.
The pads in your photo don't look bad - I'd leave them for now, and check again in Spring.