I don't think I've ever spent more than an hour there. HOWEVER, there are many, many other interesting attractions in the Black Hills. I probably wouldn't recommend any of them OVER the Faces, but several are at least as enjoyable. A few in no particular order:
Crazy Horse Monument
Custer State Park (Wildlife Loop)
Bear Country USA
Reptile Gardens
Devils Tower Nat'l Monument
Badlands Nat'l Monument
Mammoth Site
Jewel Cave
Wind Cave
Deadwood
Google Black Hills attractions to find others that might interest you. If you've never been there, a couple hours isn't enough time, imho. Take 2-3 days without the dog and enjoy.