How often you play, how you play, and the conditions you play in, will drastically affect this. However, we’ve found on average, most people change their strings every 3-4 weeks.
If you’re on tour, you’ll probably want to change them every couple of nights, especially during inclement weather. If you have a big show coming, it’s always a good idea to throw on a new set of strings for optimal tone and to prevent string breaks and tuning issues. If you're a bedroom player, you'll be changing strings closer to once a month.