On the stock door speaker, or for aftermarket? With the factory mounts you dont have much more than 2 inches of depth, but you can cut the back off unless you're going to make mdf rings.
Using a 3/4"mdf ring or the cut door mount you can go to about 2.5-2.7" of depth. More than that and you'll need to space out the mount from the door with plastic spacers or more mdf rings.