How long can mayonnaise sit out before going bad?!
Answers: I think everyone esle missed the main part of your question. I would say that as long as the sandwich was in a cool place, and in the sun or in a hot car etc... it should still be good.
Most mayos don't have to be refridgerated.
Check the label it will say.
The common misconception that mayo must be kept cool dates back to the days of homemade mayo. Today's commercial pasteurized mayos are very safe.
Mayo, once opened, must be kept refrigerated. The max to leave at room temperatures is 3 hours.
If it's store bought, then it was sitting out long before you bought it and will last a very long time. Check your expiration date which in almost everything today, but I seriously think you will use it before it expires. I buy the normal sized jar and even for a single person, I still use it in plenty of time to beat the posted expiration date.
Once a container is opened it shouldn't sit out at temps of more than about 42 degrees F for more than 4 hours. At room temp, much less. That includes on sandwich. It makes an ideal medium for the growth of bacteria. I don't let mine sit out that long.
In other words, toss it.
My mother never, ever refrigerated mayonnaise. Even after I showed her where it was written on the mayonnaise jar. And my three siblings and I lived to tell the tale.
No, I keep my mayo in the fridge-but i don't freak if it sits out overnight!!