This is my personal method, and I have not lost a shrimp when adding to a new tank for many years now.
With my method of drip acclimation, the answer to your question is ... there is no time limit.
Drip acclimation based on the 'time' factor is fraught with inaccuracy. So the answer is ... drip acclimate until the original water (bought from store or anyone else) matches your tank water. How long will that take? As long as it is needed ! 600TDS down to 120TDS could take 24hrs, or 30hrs, or whatever. It doesn't matter as long as it gets down to the same water parameters of your tank. You only need to test TDS, which is easy with a TDS meter. Because by the time the store bought water matches your tank TDS reading, all the other parameters will be the same. It doesn't need to reach exactly 120TDS either, you can stop at 125TDS for example. But closer to your tank parameters, the better.
One caution, since the new shrimp will be in a separate bucket or container, you need to make sure it will not fill up and overflow if you leave it unattended overnight.
Second caution ... temps. Since the new shrimp will be in a separate bucket potentially for many hours, you need to ensure the water in this bucket is not going to get too cold or to hot. This is particularly troublesome in cold winters or hot summers.
Hope that helps.
JayC