I think technically as long as the pH is below 3. But that's in wet form, like in a carboy. Dried up on the surface, it's hard to say. Probably as long as it takes dust to settle, and begin harboring bacteria.
To be safe, I'd just sanitize everything before you need it.
when i store my carboys i put half a cup of starsan in them, cap em, shake it up, and store them. a sealed container would remain sanitised inside indefinately. if you are talking about something that isnt sealed, i would sanitize it again right before use.
I used to leave sanitizer in both my carboys and bottling bucket between uses. Until today. I just went to transfer some wine to a secondary and was going to mix up some sanitizer in the bottling bucket. There was mold growing on the inside. Went to get my carboy. There was a colony of mold floating on top of the sanitizer.
Needless to say everything is getting the living crap bleached out of it, rinsed and sanitized. From now on I store things dry and sanitize before use.
Everyone's situation is different, but trying to keep my equipment "pre-sanitized" didn't work too well for me.
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