Asthi Visarjan should ideally be performed as soon as possible after cremation. Traditionally, it’s often done on the 3rd day, but if circumstances prevent this, it should generally be completed within 10 or 11 days. If there’s a longer delay, it’s advisable to perform it after completing the Tirtha Shraddha. There isn’t a specific “inauspicious” time; the key is to perform it respectfully when feasible