It could work within a few miles or a few minutes sitting in the driveway... but I would give it a few hundred just in case.....
Yes, I always drain initial treatments out and put in new filter and fluid just on principle after a few hundred miles. Then you can put some in and leave it if you want to in the new fluid... and clean filter...
Find out how many fluid ounces your trans holds.. and mix the proper strong mix... my wagon took two quarts...follow label directions... they have tested these things and want you to be successful....