I change around 22 gallons in my 90 gallon every month. I use a tapwater purifier that usually takes about 2 hours to filter the 22 gallons. No big deal because I'm doing other things while that is occurring. I add Instant Ocean salt, mix, get salinity right and check PH. I then drain my water from my tank and add the mixed water immediately. From the time I get through mixing my new water, it's in my tank within 30 minutes.
I did this for a few months before I knew about waiting a day or so I know it is probably not the best way and I don't recommend it. If I knew you should wait, I would have done that from the start. But I have done this every month since Sept 2000 and so far, no problems. No ick, never lost any fish and everyone is out and about immediately after the change and seems happy. I have had one case of ick in the past year but it was due to a new fish stressing out when I added it to the tank.
Again, I agree that it is better to wait a day or so, but so far this has worked fine for me.