How Often Do You Sanitize Your Freshwater Tank?

[BaconLover]

Newbie question here - but how often do you sanitize your freshwater tank? We bought our TM in February, and I did it back then, and we tested out the tank while it was in our driveway for a couple of weeks.

We emptied it then and haven't used it since, but I'll need to use the freshwater tank for a trip in June, and I'm wondering if I should re-sanitize it before that trip? Or do you normally do it at the start of the season, but is that only if you're using it on a regular basis? What if you sometimes don't use it throughout the months?

I know it's probably safer to just sanitize it, but that 40 gallons of water is a lot to end up wasting, and I live in California where we need to conserve water (although I would just end up watering our plants with it after the chlorine dissipates).