Just take a sharpie marker and add 25 gallons and make a mark right at 25 gallons. You will have to add a little water each day to keep it at the mark.
If your res is kept up on you can go almost 2 weeks before a res change.
I was collecting rain water and was fine with that until I learned a thing or to about it from the forums. You would be better of getting an ro system for about $200 but then again you mentioned you use well water so I would pull out the tds meteor and check the ppm of your water source before I bought an ro system if its high you will have to change the filters once a year maybe sooner I cant say for sure as I don't know what your water quality is to begin with. If you have anymore questions I will be more than happy to help. Good luck and check out my grow log to see it in action.