As to what age is it appripriate to leave her alone...
Well, unfortunately, there's no magic age. It's different for each dog. My 2 year old, Sable, can be loose in the house for short periods during the day and all night. She's still crated when I go to work.
My friend's Xena was good from the time she was house trained. She swears she never touched a thing! Another friend's Remmington was about 18 months, when he no longer got himself into trouble.
You can test her anytime you like, but I'm guessing if she's eating couches you might be in for a longer haul, like 2+. It doesn't hurt them to be crated. I find both my dogs generally sleep when I'm not home, considering all the stretching and yawning that goes on when I get home.
If you really insist on giving her more space, can you baracade her in a "safe" kitchen? No chemicals, cleaning products about? Nothing to chew? No garbage? Nothing on the counter? Or in her case, perhaps no chewable furniture?
Curtains?