I have recently done similar on several cars, and it's my understanding that driving it around for a short period, and using all the gears, will mix it up and dilute the new with the old. 15 minutes will probably be sufficient, but I'd opt for a day or two of driving for a much better mix and to listen/feel for any issues, and check it when it's hot and running, per the manual. I found that I somehow underfilled mine on one of my iterations, so measure carefully but also check and double check.

It's a point of diminishing returns, and there's formulas on the internet that can help you. If it's a 15qt capacity and you extract 1/3, the formula is something like this, and you can see that after #3 you are mostly replacing the new fluid to get some of the old. This is in theory assuming they mix very well.

# New %

1 33

2 56

3 70

4 80

5 87

6 91

7 94