Depends how hard you play (stretches them and they lose elasticity), how acid your sweat is (oxidises them, ie dullness), and the environment you play in (humidity oxidises them, smoke adds further gunk).
Some people used to boil their strings if they were really broke and couldn't afford new ones.
New strings are generally considered very important to sound your best, as they tune properly, and intonate properly, and 'cut through'.
However, it has been known for top records to be recorded with crappy strings, or crappy guitars, or both. Some top players also prefer slightly worked in strings to record with, as they rattle less and feel better under the fingers usually.
The only reason I mention recording is that it isn't until the red light goes on, and you put down a take that you realise (if you have half decent musicianship) how out of tune you are etc.
Recording is the ultimate miscroscope as far as your sound is concerned.
If you are a little out of tune, you'd better be playing right in the groove, with tons of feel, and a Neve preamp ;O)