I was thinking about filling my growlers with starsan and leaving them full until I’m ready to use them. Anyone do this? Will the statsan go bad?
No as long as the pH is below 3. Making a batch of star San with distilled water will give you the greatest life. I mix mine and store in 1 gallon water jugs. Growlers are too precious and should only hold beer 
If you are filling them just to keep them sanitized until use I don’t see a problem. It is hard to hurt glass except breaking it. I’m not sure it’s worth the trouble though. Sanitize, empty, let it dry and cap it then it should be fine. Honestly my growlers get cleaned, sanitized and drip dry on the inside then into a kitchen cupboard uncapped until use. Never had a problem.
If it is to save the Starsan then holaday1185 answered that.
I just realized I misread the question. HD4 is correct. Clean, sanitize, and let air dry.
I keep a 32oz growler filled with Star San from my last batch. It’s great for when you have to replace a blown airlock. I also use it to sanitize the flask for the next batch’s yeast starter. But it’s not just to keep the growler sanitized.
I do notice a build up of gunk in the growler after a time, originally I thought it was something precipitating from the star san itself, but it may be from the tap water.
