Direct Answer: It is ideal to change the strings of a guitar once in 3-4 months as per the technician who does the job. The same answer I got from seasoned guitar players.
This is important because the strings of a guitar get rusted and there are chances that they get cut while you are tuning or playing the guitar. Once I was trying the guitar of my cousin and suddenly a string got cut and pierced the skin on my hand. I just imagined if the string came anywhere near my eyes. So it’s always safe to change all the strings once in 3-4 months. Keep a reminder when you change the strings to the next 3-4 months.
