How much water remains, depends on the drier device in use. For low pressure use (≈150 psi), mechanical dehumidifiers (refrigerators in reality), can virtually remove 99%.
Desiccants (like silica gel) can do almost as good, as long as the desiccant isn't depleted. Proper drying requires a large amount of surface area, and the more surface area, the more efficiency they can be. The simple home brew one I made turns from blue to pink in about 5 fillings from 2,500 psi to 4,500 psi. It contains about 3/4 cup of silica gel. It takes about 5 minutes in the microwave to dry it back out. Incidentally, silica gel is reusable hundreds of times. Some of the other materials often used, like calcium chloride, don't work very well, and are a real mess to deal with over time. By the way, calcium chloride is what is used in most PCP-rated, high-pressure dryers. It is one of the reasons they aren't rechargeable.
Static driers (the one which look like an automotive gasoline filters) aren't very good, but they're better than nothing.
There is one alternative—dry nitrogen. Once you buy the gauges, hoses, etc., a tank refill will last a 100 or more airgun refills (depends on the pressure preset). I'm not into dry nitrogen, but some of the guys here are, and they're the ones to ask about its efficacy.
