Possibly indefinitely. If you wish to maintain a lower fc you could lower your cya level (as 70 is high if you don’t have a swg) so u can get accurate ph readings or invest in a ph meter (they are unaffected by fc levels higher than 10ppm). Remember that on the
FC/CYA Levels the normal minimum fc for your cya is a “Do Not Go Below” value - ideally you should maintain the top of the “Target” fc level so you never risk getting to /below min. So for 70 cya (in a chlorine pool) that would be 10 ppm anyway. MA maintenance minimums aren’t that much higher.