2005 SD Optima temp @ 96-99 degrees, inside. I add about 1- 1 1/2 " every other week, part of the routine. Not sure about winter yet, as this is our first with the tub.
Now I have to drag out a hose, connect it to the outside facet, purge out old water, then drag it into the room from the outside door. Adds quite a bit of time to my weekly maintenance. This is going to suck come February!
Someday when I add a water tap located right in the spa room; I'll probably top it off every week.