It takes 3714 high alchs to get from 66 to 70 mage. Each high alch provides 65 exp and you need 241373 exp to get from 66 to 70 mage. I've also included a link to the calculator I used.
What pressure is available? What size orifice , How long are people in the shower? 1 minute or several hours? This question is like asking "how fast can a car go" without knowing HP etc..