Siena was crated during the day (when we were not around to supervise) and at night until she was 10 months old and we were pretty sure she would sleep through the night. Then we left the crate door open at night with the rule that as long as she behaves, it would remain open.
At about a year old, we left her for small amounts of time during the day out of the crate (started with quick 15-30 minutes at a time on weekends) and since we didn't have any issues, we started increasing this until it was at about 4 hours.
The next step for us was during the weekday. She would get good exercise before we left for work and left her out of the crate for half a day.
Maybe we were lucky, but we never had any issues with her being bad. I think once we left her for too long and made the mistake of leaving the bedroom door open (the bedrooms are carpeted) and she urinated on the carpet, but it was OUR fault. Since then, we close the bedroom doors when we leave, just to be sure.
Twice when she had very bad diarrhea and could not wait the four hours in between she went potty in the hallway, but I couldn't have blamed her cuz it was water.
She's always been good at waking us up in the middle of the night when her body was 'off' to go potty, but other than that, it's been good.
We still don't leave her for more than 4-5 hours at a time without a backup plan. She always seems to be on her bed when we leave and when we get home and never gets into anything, so we are lucky.
However, we always ensure she has lots of exercise before we leave so she is tired and good