Ours are crate trained, and have been using their crates since we brought them home at 12 weeks old. They are now almost 2 years old and they spend about 8 hours a day (when we're at work) in there with an hour break for lunch, and occasionally some time on the weekend when we're running errands. They don't seem to mind because we give them treats anytime they have to go in there. They practically kick me out of the house in the morning so they can "go to bed" and get their treats.
Here's a picture of their daytime crate - it's wire, and 24" by 36" for the two of them. At night, they sleep in an 18" by 24" crate of the same style. We don't give them treats at bedtime, but they still hop right in and are comfortable in there. Occasionally they'll go in the crates for naps without us making them go in there, so it can't be too horrible for them.
