It really depends on what you want out of the string, and the same advice goes for most stringed instruments.
For the best tone, many performers will change strings before performances, with enough time to break them in and have the tuning set. I’d say if you can hear a degradation in tone, then that would be the time to swap them out if you were going to be playing for an event, etc.
For practice purposes, I leave mine on until they break. Shamisen strings aren’t easy to come by outside of Japan, so I try to make my supply last as long as possible. If I can manage it, I’ll even let the string out and see if it’s long enough to re-tie and keep playing. The only reason I’ve switched them out before they’ve snapped is to change up the gauge or material of the strings.
If you’re lucky enough to be where you can run to the local shop and pick up new strings, or if you order in enough bulk to manage it, then you can change them whenever your tone starts to degrade, if you want to. If not, or if you’re just putting in a lot practice, there’s no harm in going until the strings give out on you.