I've been using Houghton Hocut 795 since I got my Haas mill (used) about 4 years ago. I have never changed all the coolant in this machine yet.
I might also not run the coolant for a week or two sometimes, but I do have a skimmer running daily for an hour. After a long sit, the coolant does have a mild 'stinky socks odor' when it is used again. But, the smell dissipates once the whole tank has been aerated (through use) and is quite satisfactory to continue to use.
I've used other coolants in the past, and experienced poorer results and would have to replace the coolant. But this Houghton stuff seems to be pretty good and does not grow a skin. I do not use a bacteriacide, as I don't want that stuff soaking into my skin when I'm working with wet parts or chips or washing down the machine and getting splashed on. I don't like to use bleach except in a clean-up stage when redoing a machine. I don't know if one can trust that the bleach only makes it bad for bacteria, for it may react with coolant as well.
The stuff that the skimmer picks up, I wick the oil off the top of that with absorbant paper towels. Then, what remains gets used up in the saw, or for drilling manual on the drill press, places where I normally do not keep a tank of coolant on the go.
First you get good, then you get fast. Then grouchiness sets in.
(Note: The opinions expressed in this post are my own and are not necessarily those of CNCzone and its management)