There was a similar post recently.
StarSan will last a very long time. I usually replace mine prematurely simply because it eventually gets shmutz in it. But it's sanitized shmutz. I wouldn't necessarily need to.
I had a 5 gallon water bottle that I would top up with a couple of gallons when getting low. It was never empty for over 5 years. Get some pH strips. Anything 3.0 or lower and it is good. Even if it goes cloudy in some water chemistry it is still good if the pH is right.
I have a 5 gallon bucket that I keep mine in. I replace it when it gets schmutzy (terrific word BTW) as well. Instead of pH strips, invest in a good pH meter. You can find them for less than $15 on Amazon, I think mine was $12. Not only is it great for making sure my sanitizer is still good, I use it to test my mash pH. And the first week I had it, just about anything else.
If you didn't use distilled or RO water there may be some white chalky sediment on the bottom. I usually just slowly pour the clear Starsan on top into another bucket, leaving the last bit with the white residue behind. Never had any trouble that way, it sanitizes just as well.
+1 to pH test strips or a roll of cheap multitest pH paper that includes a pH 3 indicator color.
I just keep Star-san in a bucket and when I dump it in a carboy or keg to sanitize it, I reserve the last inch or so that sometimes looks crappy.
Eventually the amount of star san in the bucket gets lower and starts looking crappy and then I'll get rid of it.
I never check the PH, don't worry about it and haven't had any problems. A 32 oz bottle of Star-san than I bought 3-4 years ago is still about 1/3 full
Wait wait wait. Star-San can be (re)used for months and months? My hard water also turns it cloudy immediately but I guess I never thought the product had that long of a shelf life when mixed.
So I can mix up 5 gallons and use it multiple times? This is good to know, I wonder how many new brewers assume like I did you mix up a batch for brew/bottling day and then that's a wrap on that star san.
I learned very early on that Starsan can be reused, saved almost indefinitely if the pH stays at 3 or below. I got a 16 ounce bottle shortly after I started brewing July 1, 2011. I still have about 1/2 gallon from that first bottle. I had great water where I used to live and it never got cloudy. The water here makes it go cloudy on mixing. I have a 32 ounce bottle now. So 7 years on the first bottle, 100 batches. I expect I will get 200 more batches with this bottle and maybe 14 more years before I have to buy more.