A couple of days should be enough. If not, as the vinegar gets dirty, I have strained the vinegar and placed it back in for a few more days. I have had real good luck with making motorcycle and outboard motor tanks CLEAN.
Well, for a barrel de-blue it was like one or two days.
Guess you can drain and refill periodically to check how things are going.
Main thing is that it's not some instant reaction so don't worry that it will destroy your canteen in a few minutes.