I'm sorry I don't know the EXACT place I recently read this...but I've been interested in this myself lately. We've had Daisy about a month now, and she is roughly 3 months old. What I read said that it is OK to leave your dog in the cage for as many hours they are months old.
So, 3 months = 3 hours
8 months = 8 hours
This may be TOTALLY not the norm here, but like I said, that's what I recently read. I personally would feel bad leaving them alone for longer than an average work day no matter what the age...and we have not left her alone for more than a few hours so far ourselves. But think about all of the people who own dogs that DO have to leave their dog in the cage for the work day. I'm sure the dogs are OK. Just give LOTS of attention the hour before you leave, and give access to some food/water. But if you have the opportunity to keep them out of the cage a bunch, than that's awesome!!
