If you have no choice but to drip, the best collected wisdom says no more than 30 minutes: your mileage may vary with temperature (hotter means faster), time (distance from the lfs), etc. My advice is, if you have to drip:
1. The death clock does not tick until the bag is opened. Correct your temperature first by floating (15 minutes will do it under most ordinary circumstances) your unopened bag. Now---if you're warming it up, remember warmth accelerates chemical processes. So:
2. you are now warm. Open the bag and start dripping: reduce the water volume in the bag a bit and look at the clock. Say you've got the average water bag, and the rascals shipped you a fish at 1.021, and you're 1.026, a .005 difference in salinity, and you've got 30 minutes. You only need to change that salinity by .004 to be close enough for a makes-no-difference. So proportion out your drip time by segments of time (4) in which you have to up that salinity by .001 every 7 minutes or so. So just start dripping and measuring, and if you get to your first 7 minutes and you're not at 1.022, pick up the pace of the drip. If you reach 28 minutes and you happen to be at 1.024, and I had to choose between 5 more minutes of drip and getting the fish out of that bag water, I'd move him on over and get him out. Unless he's super delicate he won't die of a .002 difference: currents waft by in the ocean now and again---fish can vote with their fins in those instances, but your guy still is going to survive it.
The deal with ammonia is that it's toxic, and by the way it kills, from right away (if awful) to 3 days on, indicates to me that it blitzes the kidneys: takes a while for blood toxins to build up to a fatal level, if the kidneys are out of commission or nearly so. Hence the numerous accounts of fish that made it 3 days and then keeled over dead without a mark. The unfortunate owner doesn't know what happened: the fish was eating, no sign of ich, and everything was fine and now it is dead and no mark.
If you've got the freedom of a qt tank, and by that I don't mean a cycled tank, but a barebottom glass box or bucket that can hold a fish for a while for observation for parasites and to get them eating properly....it's just so much easier to adjust the tank instead of adjusting the fish. You then set about adjusting the qt to match display tank salinity by just letting evaporation handle it, and you're good and the fish has never had a moment of stress about his salinity.