I am embarrassed to admit that I have a set of strings on one of my "go to" guitars that must be 6 months old. Daddario EXL 110's to be exact. No, there aren't bright any more. Yes, they are goober'd up with flat spots from the frets. But they just keep taking a beating as I stretch them and use my tremolo with reckless abandon. They are mounted to a Fender 50th Anniversary Deluxe Stratocaster.
However, I generally change my strings before every gig. There after, if I am just practicing the guitar, I gauge my consumption based on hours on the string. If I am practicing a hour or more a day then I change electric guitar strings twice a month. Acoustics string definitely are replaced when they tarnish but generally I apply the same rule as my electric guitars. Never gig with old strings!
Always use a string winder.
Stretch your strings! Try this product - Stringstretcha (review) [purchase direct]
Install properly - Instructions link. Wear EYE PROTECTION!!
Beware that string gauge change will effect your intonation,neck bow, and pickup output at the least.
I usually buy my strings from Just strings.com but I will buy my strings and various assundries from Guitar Center or Musiciansfriend if they have a great deal.
Now it's time for me to change those old strings before one snaps and cuts me!