Took me 2 years of nights and weekend work to finish mine. I also have a home theater, pool/game room w/wet bar, candy counter, dart area, and small air hockey, plus my one MAME cab. Since I didn't get the pool table yet I'm now thinking I would get more use out of a nice poker table...
Also have an office down there with a 75 gal. aquarium in the wall and lots of synthesizers I never use sitting around.
Anyway, 15' isn't a lot for the depth. Will you have enough room to move the couch back from the screen (are you doing a TV or a projector?) and stil have room for arcade cabinets behind it. What size screen are you putting in?
I've got 100" 16x9 and in the first row of seats the viewer's head is about 11' from the front of the screen, personally wouldn't want to be much closer than that with a screen this big. That would leave about 3' between the back of the chair and the wall in a 15' deep room.
My theater dimensions are about 19'x11'.
You may want to measure everything out before you finish your wiring. Also I HIGHLY recommend you run conduit of some type so you can upgrade speaker and AV wire upgrades incase you need to. I've had to upgrade to DVI then HDMI cables to my PJ and somehow managed to put a nail through one of my speaker wires so had to run another after everything was finished.