If you take care of your ukuleles, including proper humidification, an initial setup should last a while. If you play hard and frequently, your saddle may wear down and need replacing. This guy says he takes an electric grinder with him on tours so he can setup a new saddle if his current one starts wearing out.
It really depends on many factors. If it is a laminate uke and you live in an area with stable weather and climate then it can be very long like decades. If it is a lightly built solid wood uke that was built in tropical climate and then lives in a seasonal climate it may be more on the order of months. Even if the new environment is climate controlled it is likely that the uke will change to adjust to it and then seasonally even though changes may not be extreme. Best time for setup is usually several months after purchase when you can be sure that the uke has equilibrated with environment and you had time to play and note areas that can be improved with set up.