Too expensive to use that way -
It's not a cleaner, it's a sanitizer - so a bathtub full is a waste - you're going to let it all down the drain.
If you mix a batch - even a gallon in a jug, then pick up a small sprayer from the shampoo section of wallyworld, you can spray down the items that are clean and need sanitation. You can pour a bit into your carboy/pail and swirl and swish.
it's a wet-contact sanitizer. Clean stuff can sit soaking awhile, or not - but just before you use it, (tubing, autosiphon, pail, etc.), get it wet with starsan.
I mix up 5 gallons and store it in an old corny, and just use what I need. My 32oz bottle of concentrate may last the rest of my life...