Definitely not longer than half an hour. No matter what.
DO NOT DRIP ACCLIMATE. Find out the salinity of the shipper, pre-set the qt to that salinity, float the sealed bag for 15 minutes to adjust temperature, open and test the bag. If within .002, you're good to put him right on over to the qt with no drip acclimation whatever, because the SALINITY is the same in bag and qt. Then over the next 4 WEEKS use evaporation and topoff to re-set the qt salinity gradually to that of the DT.
For a fish to suffer a rapid salinity fall is ok. For a fish to suffer a rapid salinity rise is dangerous.
What is even more dangerous is that if there is ammonium from fish excrement and breathing in the bag, opening the bag will convert it to ammonia, which will kill the fish within 3 days from organ damage.
This is why no drip acclimation. Drip Acclimation is still the only recourse you've got when your info is wrong and the salinity you were told is dead wrong. That's why you test the water. Be sure. But if you've done your homework your fish doesn't need to undergo that dangerous adjustment. He can go straight into safe, clean water.