[P-did]

I would say two or three hours max. Keeping a dog in a box for much longer can lead to serious behavior problems.

For our puppy, we set up a play pen with a tent (her den) and a little box (with dog litter) at the opposite side. She has enough room to jump and play while we are at work, and always uses the box. It's much easier for the dog to distriguise the box from other places they should go like the carpet.

No matter what you're plan, crate train, IMO, is only met for people who are home often enough to release and exercise the dog every few hours. For those of us with full-time jobs, I believe a playpen is the only option that is fair to the dog.