+1 for Elixir strings. I've used them on my acoustics for years. They are more expensive (or at least they were last time I looked), but they sound good and last longer.
Strings deteriorate with use and over time. There's no exact number of days or weeks as it depends on several factors, but you'll know they're gone when you get a dull, thuddy sound rather than a bright, pingy one.
(I'm good with technical jargon, me. )