Hi, NaH20-
I do 25 gallons monthly on both of my 125's, after using a powerhead on the rocks and stirring up the substrate.
I haven't seen any difference from the 6 gallon weekly changes I used to do, except convenience!
I use a big rubbermaid roughneck trashcan to mix my salt 48 hrs in advance, then I just wheel it over to the tank and pump it in! (i use another of the same cans to drain, down the sink). I also use one of these cans to collect and store ro water, I think they're indispensable and wouldn't dream of being w/o them now.
It has cut my w/c time down to 10 - 15 minutes from the tedious and time consuming labor of using 5 gallon buckets.
I also disassemble and clean my skimmer and prune refugium algae, etc on my monthly w/c days.
Seeya, Becca