For how long can you let dough sit there to rise without yeast? I read that you can make a dough without yeast and have it sit at room temperature for 12 hours and it will rise due to bacteria found in the air.

It didn't rise after 12 hrs, so I was wondering if I could let it sit for another 12 hrs to see if it eventually rises? Would 24 hrs be ok or that's too long?
I am not sure where you read this and it is a reasonable expectation that it would work. This is however a classic example of recipes authored by someone that worked in a certain climate, season, location and other specifics that are not always going to work everywhere else.

As already pointed out by @kettultim, natural yeast count in your environment is not something you can take for granted. It is entirely feasible for a dough to sit around for a couple of weeks, get moldy and not rise even though you had tried the exact recipe before at another time or place.

If you had it working before and you liked what you had, keeping a bit of the fermented dough and nurturing that as a starter for later use is always worthwhile.