Hmm I let Mahina chew on them all day. Well, they are left out all day and she chews occasionally. Usually when she wakes up or if we're just watching tv.
It takes her on average no longer than a week, sometimes she'll chew one down in a single sitting.
I give them to her or else she would chew on my door and my dresser. Plus, I really think it helped loosen all her teeth, she didn't have to have any pulled at her spay two weeks ago.
I don't know if I'd let mine have it past a few weeks. Also, I buy about 50 at a time from AMD and they are in plastic bags in my closet. They haven't changed appearance, and I've had them since I believe November so I don't think refrigeration is necessary.