Can anyone tell me how long the Star San can keep once mixed and still be effective?
I believe that is going to depend on the quality of the water used to mix it. Distilled water will last longer; harder water with more stuff in it will last a shorter amount of time. Once it begins to go cloudy (which to me indicates that chemical reactions are taking place), I dump it and mix a new batch. I don’t believe there is a hard & fast rule on the stuff.
Hope that helps.
Using tap water, I used to get a week or ten days before it got cloudy. Now I use RO water to make it and it easily lasts 6-8 weeks if I keep it covered.
I use tap water/city water to make mine. I don’t usually keep any on hand though. When I started brewing 1 year ago I bought one of the smaller bottles and just had to re-order star san last month. On brew day/racking to secondary day I only use 1 gallon of star san mixture which is only 1/4 oz or less of actual star san. I mix it up in a gallon jug, pour it into the bucket or carboy and go from there moving it around to a spare bucket to sanitize the other things I’ll need. Then I usually dump it out.
It may sound wasteful but why the heck would you need 5 gallons of star san? I’m new so I have no clue. Maybe some do want that much, but for me, a gallon at a time works and in the past year and 20 batches I haven’t had a problem yet. Just make sure your mixture comes in contact with everything and when in doubt, make it a bit stronger than not as strong.
For prolonged use I would suggest distilled. Ph is also important. I believe you want around 3.0, don’t quote me though. Denny’s kept it around for two years with distilled water and Old Stoner voodoo.
Prepared with the tears of a virgin, eternity. Good luck with that. 
cloudy/not cloudy is not what matters. What matters is the pH. You want the pH under 3. If you have really hard water, it could be cloudy upon mixing - that does not mean it won’t work. Like I said, pH is the key.
As mentioned, mixed with distilled water, it can last almost indefinitely. I keep a big tub in my brewroom full (10 gallons) and constantly throw stuff in and out of it. I brew a lot, so, for me, it is really a matter of convenience to have it sitting there and just dunk tubing, buckets, gallon starter jugs, use a pail to fill kegs, etc. I generally make a new batch once a month just to be safe and because I end up with floaties, etc. over time.
If you use distilled it should last for months without any issues. As others have said keep an eye on the pH, as long as its below 3 you should be goodto go.
When my StarSan bucket begins to accumulate a layer of “crumbs” on the bottom, I’ll decant the StarSan into a clean bucket, dump the crud and spray out the original bucket, transfer the StarSan back and top off with a bit of DI water. If the pH shifts up, just add a teaspoon of fresh StarSan - keeps a long time like this at minimal cost.
As long as the pH is 3 or below, you’re good. A 5-gallon batch usually lasts me 3-4 months. Even then, the pH is still fine. It just starts to get a little cruddy from constant reuse and dipping equipment in for sanitization.
