Minimum Requirements For O2 Priority PAYG

I was thinking of getting my Dad to switch to O2 in order for him to have priority moments. He is pay as you go and hardly uses his phone. The odd text and call back, he usually uses wifi and whatsap to communicate. In order to get and keep O2 Priority... How much do you need to top up? How often do you need to top up? How often do you have to use your phone?