They do get used to it. 20 minutes really isn't that bad. I've got a basset hound who's 10 and was never crate trained before I got him. He still whines and barks in the crate at night. It's part of his routine, but he's definitely a lot better. We have a night time routine. The dogs come inside and insist on patrolling the floor for any food that may have fallen on the floor during the day. Then I give them the crate command. They get in their crates and the hound goes nuts banging his tail against the crate and barking his head off. I turn off the lights and tell them good night and the hound lays down and barks a lot less louder for about 5 minutes or so.