I like the occasional Negroni or Manhattan, but I'm not a heavy drinker. As far as I'm concerned liquor is pretty much immortal, but what's the rule on vermouth?
Opened, at room temp: A week or two without serious flavor changes.
Opened, but kept in the fridge: a month, at least. (I'm still working my way through a bottle of dry vermouth with no noticeable degradation in flavor in ~3 months, ymmv of course).
Unopened: A year+ at room temp.
Source: Serious Eats experiment