I got Dixie when she was 6 weeks old. (too young, I know)
I'm a single mom and work a full time job. I often worried about Dixie being in her crate for extended periods of time. Dixie is in her crate for 4 hours in the mornings, and I come let her out at lunch. My kids get home after school and let her out right away too. So, she's in her crate for a good 8 hours a day, with one break in the middle.
She is 12 weeks old now and no longer pees or poops in her crate. Also, at night if I don't feel like sharing my pillow...I'll say "C'mon Dixie, it's time to go night-night in the crate" She'll get up and go straight to it, with no problem at all.