From everything I've read, rule of thumb is to try not to leave your dog alone for more than a contiguous 8 hrs. More than that, and they can be holding it for an unreasonable amount of time. Since you're close, and can let him out at lunch, I say go for it.
If you went 10 hours, it wouldn't be the end of the world, either. I've done it before, I just try not to.
Is the dog crate trained? It's kind of a bitch to crate train them if they're older.
If I'm going to be gone more than 8, I'll generally leave mine out of the crate, and he's happier for it. That way if he REALLY has to go, he doesn't have to sleep in it. It doesn't happen regularly, but I've come back a couple of times after an extended leave with a poo pile by the back door (where he gets let outside).
Plus, when he's left out of his crate, he expends more energy barking at shit.
Hint to keep them from chewing shit up: ALWAYS give them real dog toys from the store - don't give them old socks, plastic bottles, etc... They can't always tell the difference between the sock "toy" you've given them and the real thing from the laundry hamper. Also, have a crapload of toys spread around and available. That way they always have an outlet for their chewing close by, and a nice variety.