I would recommend changing your strings every two or three months. This will depend on how much use the string is given. The following advice will help tell when it is time to change your strings.

• When string start to have tuning problems—change your strings.
• If you instrument sounds opaque and starts losing its resonance—change your strings.
• If the strings begin to oxidize—change your strings.
• When the strings start to show sign of wear—change your strings.

The duration of the string may varies from person to person. Not only because of the use, but by the chemicals that the hand liberates when playing. How strong the string is being pluck may be factor to determine the duration of the strings. The following recommendations are to be followed for a longer life of the strings:

• Wash and dry your strings before playing.
• Clean the strings and fret board after every use.
• Not to play with humid hands or when having a snack. 
• Use stainless steel string to play.

Don’t forget to have an extra set of string available in your case—strings break when you less expect.