I usually exchange the nephrostomy tube every month. Keeping the same catheter without exchange would result in a higher incidence of bacterial colonization, more recurrent UTI, stone formation along the catheter and catheter blockade preventing adequate drainage.

every 6 weeks in our centre.

Every 6-8 weeks under complete aseptic technique, unless it is obstructed. This is to avoid encrustation and UTI.

up to three months in uncomplicated cases.

Three months in my Institute unless complications occurs. 

Up to 3 months in uncomplicated cases. However a guideline on this topic hab not yet published yet. Time intervall depens on material of tube and factory reccomendations.