Maybe Desmond was an odd case, but we could leave him for just about 8 hours when he was 5/6 months old. I have school, so I am out for 8 hours every day. He has never messed in his crate the whole time we've had him, and is always fine when I get home. We go out and potty IMMEDIATELY when I get home though.
My "rule" has always been they can hold it for an hour for every month old they are, and one hour more. So Lucy is 5 months old, she should be able to hold it for about 6 hours.
If she has a history of not being able to hold it for very long/messing in the crate, I wouldn't leave her for that long though.