[JoeKool_42]

I will be patching a large whole that used to be a window between two interior rooms. The drywall appears to be almost an inch thick. I didn't think drywall came that thick. What thickness should I get to fill the open space?

 

[ClarenceBauer]

Looks like you may have a Gypsum board or Rock Lath with a Plaster applied over it.
Just use 5/8" drywall than apply a setting compound over the drywall out to about 1/16" from the existing finish, than use a finish coat material of your choice.

 

[dj3]

You can use the standard 1/2" drywall, then a second layer of 3/8" to get something to match what you have.
Just use longer drywall screws for the second layer and make sure they go into studs.

 

[mark sr]

I'd use whatever drywall thickness I had handy, shim it to be just shy of flush and then finish with a setting compound.