What's the shelf life on Marsala Wine?

Marsala, like sherry or Madeira, is a good wine to keep on hand for cooking as it has a long shelf life. Also because it's delicious.

Marsala has a high enough alcohol content to remain usable for months. Fortified wines were originally produced for just that reason, to survive long voyages by sea. The wine will slowly lose quality, a process that, like most, can be slowed by refrigeration, although not strictly necessary.

Vinegar can be made using wild yeast but I'm not a fan of relying on chance (especially since chance may provide you with undesirable elements as well). I'd do a little research on the optimal alcohol content for fermentation -- the high level of alcohol that preserves fortified wines is also high enough to kill off the acetobacter bacteria. In other words, some level of dilution would be required.

Marsala is a fortified wine and I don't believe it will spoil, the taste diminish after a while but you can pretty much keep it indefinitely. I have an opened bottle that I use for cooking and its 6 months old and still good. lloreen is correct if it were vinegar but I assume you are talking about the wine.

yes, it will spoil after a few days. Actually, making wine vinegar takes more effort than just leaving the bottle open. You need a "mother" vinegar culture and you need to keep it in a darkish place for many weeks. Maybe you should just drink the wine with dinner...