+1, I generally only add it to treat new water during water changes, and only enough to treat the newly added water. (ie: if I'm changing 15 gallons of water in my 36 gallon tank, I use 1.5ml of prime)
Prime is not a fertilizer or maintenance chemical. You positively should NOT dose prime without a reason, as it does reduce oxygen content somewhat.
That said, you can dose prime to detox ammonia if you have an ammonia spike. However, this use should be considered "emergency" not "preventative. Regardless, you should also be doing a partial water change at that point, and that means you'd be dosing prime as you would any other water change.
Prime also has metal detoxifying properties, but this also should only apply to new water (ie: copper from your pipes). If there's something in your tank leaching toxic metals into the water, remove it, don't keep re-treating it with prime.