I started crate training my pup 5 weeks ago when I got him at 8 weeks. Besides the first 15 minutes of whining and the occasional "I need to relieve myself whine", I've found crate training to be a welcomed experience. At 8 weeks I was leaving Sagan in his crate for about four hours at a time making sure he wasn't too uncomfortable holding his bladder. But its just out for about five minutes then back in if I don't have time to play or keep an eye on him. Puppy's can sleep upwards to 18 hours a day so I wouldn't worry too much about leaving them in the crate too long. Just make sure you remove them at regular intervals so they go to the restroom.
It also helps to feed them in the crate to make it a more comfortable space for them.