When you say "filter media", what type of media are you referring to? If a desiccant, and it is not an indicating desiccant, the best way to know if you need to replace (or recharge, if appropriate and feasible) is by weight. My advice would be to weigh out the new media when you put it in the filter housing, and also weigh out the empty filter housing and then the whole thing together. When the filter weight increases by about 10-15% of the weight of the media you put into the filter (not the total filter weight) it would be a good idea to replace it, depending on what type of media you are using.
You can't really go by the tampon filter, as those are absorptive filters where desiccants are adsorbtive. An absortive filter will only catch liquid water, and will freely pass water vapor - ever notice how those don't get wet on humid days, and that they will dry out if damp when left outside of the housing? All they do is catch any liquid water that gets through the pump. The water still in vapor form is invisible to them . . .
As to your question on whether usage of the tank and cycling it with dry air will dry it out, the answer is yes, but within limits. If you somehow got several ccs of water in the tank, it will be hard to get out that way. But what can help would be to reduce the pressure in the tank, as some more water can flash to vapor at the lower pressure. Lower the pressure to around 2000 psi, let it warm back up and then vent another 500 psi, and then refill with very dry air through a molecular sieve filter and most small amounts of water will flash off and be removed.
Honestly, that is part of the reason why so many people think they had no water in their tank when they open it up and see it is dry - even if they had a few ccs of water in a tank like a 6.8L one, the process of venting it will usually lead to the water flashing to vapor as the tank is vented because the air is so dry in there to begin with. It is all a pretty tricky thermodynamic situation to deal with and understand. The key to a good inspection is looking for signs that water had been present, not just for the presence of water since it will often be gone by the time we look in the tank.
