We're going on a long flight with our family tonight. I prepared peanut butter sandwiches for the kids, and would like to pack it already to avoid the last minute rush (or forgetting them). We're not leaving for another 12 hours or so, and it will probably be eaten a few hours after that. (I'm not so concerned about mushy sandwiches because these are little kids, they won't mind.) Bread and peanut butter individually can be stored at room temperature for a significant amount of time... Can I leave my peanut butter sandwiches out for ~15-20 hours?
I've done it before without problems. Even if they're peanut butter & jelly (something that's typically stored in the fridge), it has so much sugar in it that it's inhospitable to microbes.
If you want to play it extra safe, and the sandiwiches won't be easten shortly after you leave, you could place them in the freezer and then let them thaw in your bag.
... all that being said, if you're in the US, the TSA considers peanut butter to be a gel, so won't let you fly with a container of it. I have no idea what their rules are in sandwich form. (I've never tried it, as I typically fly w/ cold cut sandwiches, and just take them straight from the fridge before I go).
update from Erica : "I have flown with peanut butter sandwiches in our carryons. New TSA rules mean pulling it out of bags to send through the scanners, but they had no objections."
They can be considered bad when the bread becomes moldy. If it is regular bread with normal preservatives then I would give them about 7 to 10 days before not eating them. Peanuts last stupidly long and with all the sugar they put in peanut butter the microbes almost cannot grow in it. I would be more concerned about the staleness of the bread before woring about anything being unsafe.