How long can a sandwich with mayo on it last in the fridge?

If the mayonnaise is unopened it should last about a year, in or out of the fridge. Once you open it you should store it in the fridge. Mayonnaise usually contains vinegar, which helps to preserve it but you should probably finish it before two or three months.

It helps a lot if you do NOT introduce bacteria into the opened jar. So use a clean spoon, and don't lick and stick your fingers in!