If it’s been in the fridge I would say at least another day or two. My roommates and I have eaten it during that time frame, and it was all finished before we could make it extend any longer haha.
If you mean tuna and mayo combined (maybe with other stuff added, like celery or green pepper, a little lemon juice, onion, etc.), as for sandwiches, I would consider it safe to keep for three days.
Don’t ever leave it out on the table. Mayo is a notoriously fast spoiler. Stick it in the fridge, covered, in front of the other stuff so you won’t lose it..
@susanc Good advice. It could last, but why take chances. I have a cute little picnic cooler that you fill with ice, and then keep the mayo and such in.