Drip hydro advanced nutes help

So i plan on watering 4 plants in a hydro drip system using a shared reservoir in a large bin and hydroton medium. The reservoir will be about 5 gallons, and I plan on following the AN schedule using sensi 2 part and big bud only.

My question is, how often do i need to change the water in the reservoir? Is it every week like on the schedule? The schedule always has 1 week intervals reguardless of res. size so i doubt it..
Or do i change it based on the ppm of the reservoir? i just purchased a ppm meter and a ph meter as well.

Thanks!

I top up daily, check ppm,ph twice a week, and change tank once a week. It's down to how clean you want your housekeeping to be really...