There is so much variation between banjo players on so many factors, that I often stay away from rules of thumb that get too specific (such as: every ___, or every ___ hours of playing time). Like many things, it's up to personal taste. But I do think many people leave them on too long, where they sound duller, are more prone to breakage, and become harder to tune, due to the overtones becoming out of tune, due to wear of the strings.
Probably the best guide is to listen carefully to the difference between old strings and the new ones once you put them on. I personally like the sound of fresh strings. The brightness of the wound 4th is usually the first thing to go, and then the general banjo gets slowly less bright (which some people like). When it becomes hard to tune, then it's time to change. The sign of this is when your electronic tuner says it's right, but it still doesn't sound right, and the chords sound out of tune. That's the overtones being out of tune.