There isn't a single answer. For someone that doesn't have a stable tank, the answer is never. For someone that's a pro with the perfect setup, the time is incredibly short. If you are relatively new, the answer is likely in between- this is why you see rules of thumb like 6 months, 1 year, etc. Part of that time is to allow you to learn and hone your craft keeping your water parameters stable. Part of that is to allow your biological filter and water chemistry to stabilize.
Instead of asking for a definite time, I would start by testing your water regularly. If you can consistently demonstrate that your temperature, salinity, pH, alkalinity, and nutrient levels are stable, you'll take that concern off the table. That's usually the hardest part to master and the "rate limiting step". For some, that takes a few weeks/months to dial in, others much longer. Every tank and every reefer is different. Just remember, you can't wait too long, but you can certainly rush it ;)
