How long does vermouth last once opened?
Not as long as most people think.
Vermouth is a lightly fortified wine "aromatized" with such flavourings as herbs, bark, fruit and spices. The alcohol – 15 per cent in the case of Martini Rosso, 18 per cent for Noilly Prat white – helps curb decay. So does sugar, which is moderate in "dry" white vermouth and high in sweet, red vermouth. But it's wine nonetheless, and air causes oxidation, which yields unpleasant, stale flavours.
White vermouth, the style used in martinis, goes off quickest. It's best to consume a bottle within four or five months, I find. A half-filled bottle sitting around for a year is pretty much dead. You can see it in the colour, which turns darker as it ages. A tired bottle of white vermouth still makes a fine poaching liquid for fish. I've got some Noilly Prat sitting around for just that purpose (for some strange reason my consumption of dry martinis has been modest of late).
Store vermouth in the fridge after the bottle's opened if you can. Cold slows down oxidation reactions, keeping your bottle fresher longer. And look for half-bottles if you can find them, unless you live in a five-martini-a-night household.