I was gifted several Wilton stackable pans. About how much batter should be placed in each and for what cooking time times? The sizes are below.

RECTANGLE: 9 x 13 x 0.75

RECTANGLE: 4 x 10 x 0.75

SQUARE: 6 x 6 x 0.75

ROUND: 8 x 0.75

ROUND: 6 X 0.75

Side note: I usually use Duncan Hines cake mixes. I cook two boxes at once in a huge bunt pan for 60 minutes on 350F.

Thank you.

fill them about half way full and bake them till they are done -- there's no real way to say exactly how long

Being that they are only 3/4" deep will reduce the baking times most of us use.  I suggest baking them at only 300 degrees F.  It may take a couple minutes longer but worth it for a far moister cake:)  K8 is right - just fill each pan 1/2 full of batter.  I'm guessing time at 300 will be only about 15 minutes, but that's just a guess/starting point.  You are going to have to experiment since the depth is so shallow.