Todd,
I build my workbench table tops out of 3/4" plywood, two thicknesses, and cover with tempered hardboard.
I screw the hardboard on so it can be replaced if it gets messed up to bad.
The two thicknesses of ply may be overkill but it is rock solid.

If your table is going to be up against the wall 30" is about the max depth.
You wont be able to reach anything on the backside or wall.
If it will be in the open and you can walk around it you can go deeper.
I wouldn't go more than 48" in that case.

Good Luck,
KW_Counter