There are several factors that will determine the longevity of your instruments strings.  The first of course is the Quality of the string. Always go with a trusted name brand that you recognize or that you have researched. This is as easy as reading reviews on the brand Also make sure to buy your strings from a trusted source. Name brand strings that have sit in a musky warehouse for 10 years might not last as long as a set of off brand strings that are new.   
The second factor is frequency of use. If you play every day, don’t expect your strings to last as long as someone who plays infrequently.  Your body chemistry can be a determining factor as well. If you sweat a lot when you play or if your body’s PH is particularly acidic, this will shorten string life as well. Other factors will be the climate in which your instrument is subjected, is it particularly humid where you live, is your instrument kept in its case when not in use? These are all factors to consider.  Here are some simple tips to increase string life.

• Buy quality strings from a trusted source
• Always wipe off your strings when you are done playing
• Keep your instrument in its case or bag when not in use.