Another way to tell that the can juice is going bad is also color. It will start geting a slight brown tinge. Quite possibly if you mix the lemon juice into your batch it might act as a preservative and help it last a little longer. I can't say the preceding is "fact" since I'm just going from vague childhood memories which I seem to remember my mother adding lime juice to it to make it keep a little longer.
I've recently moved much closer to the farmers market where I found fresh cane juicein the states (Atlanta, Ga). On a recent visit last week I checked with the prospect of doing an MC with it in the spring but unfortunately they don't have it any more. Dunno that I could have afforded to do a full MC since it was fairly expensive at $6-7 US for half gallon but at least it would have been a nice occasional treat and change of pace kinda thing.