the filter on the left is still basically new and could be used more. the one on the right looks like its due(not overdue) to be replaced. if you want to know for sure when its time to change them. get a vacuum gauge and adapter from racor.
My primary filters looked much like the OPs when I changed them. One dirty, and the other clean. If I recall, Racor says that the filter elements do age even if they are not dirty. I don't see anything wrong with running on one of the parallel filters for normal use. This means that your backup filter should be completely clean in the even you need to use it.
My thinking (such as it is) goes like this. In normal use, the filters should not get dirty enough to cause any fuel restriction over their useful life (for me a year). If something unusual happens, such as a load of bad fuel, rough weather that might kick up debri from the bottom of an old tank, etc... then your working filter could get clogged. In that situation, I want to be able to switch over to the standby filter and have it be able to filter and function for as long as possible. During this time I will be in the ER changing out the clogged filter.
Now, lets say that an operator is switching which is the working filter on alternate uses. Then, in theory, both filters would be getting dirty at the same rate. So if something bad happens with the fuel, both filters might not have much spare filtering capability left. ie both might clog quickly not giving me as much time to change the filter.
Does that make any sense at all? I figure that I am going to change both filters yearly anyway. If one of the dual filters performs that entire time great. I see no need to switch back and forth (other than to simply make sure both sides of the system are working well).
Edit: I just did a little looking to substantiate my idea that the filter manufacturers recommend filter replacements based on time. I didn't find anything easily, which seems to me an indication that maybe it isn't an issue? I did find a good article by Steve D'antonio. In the article he recommended that you simply use vacuum restriction as an indicator for when to change the filter. In that article he mentions that the filter manufacturers do recommend at least annual changing of filters, but he points out that they have a vested interest. He writes that in his experience, he has not seen a filter fail from being submerged in diesel fuel.
Link to the article. Look near the very end of the article for his comments on replacement schedules.
So, maybe my yearly replacement of filters is overly aggressive. I think I will still continue that practice. If nothing else, it forces me to really get up close and personal with that portion of the fuel system. While I do checks on it daily, I admit that they tend to be cursory.