How long can you keep homemade mayo?

I've read that in general acetic (vinegar) acid is a better preservative than ascorbic (lemon) acid. So if you plan on storing it for longer it might be good to use vinegar in a recipe that calls for lemon juice, with the appropriate adjustment.

Great question! There's such a lot of disparity in answers when you cast about on the web. Some people and organizations say not to make it at all. I've found one Canadian government recommendation of ~a week, and that seems intuitively good to me. Sorry not to have more science in the answer.

Alton Brown's recipe says refrigerate for up to one week. He lets it sit at room temperature after mixing for an hour or two to allow the acid to denature any bacterial proteins.

I would use it as soon as possible; at most 2-3 days in the refrigerator.