Does your store have a written policy that answers this question? Even if it does, have you tried to contact the customer? Do that, and do it in writing using a provable method of delivery, and give them a definite time to pick up their art or you'll no longer be obligated to store their art. Imagine them suing you, and plan for your defense by showing how reasonable you're being before you get rid of anything.
To make sure this doesn't keep happening, provide your customers with appropriate and lawyer-drafted written policies about payment and pick up requirements.
Avvo doesn't pay us for these responses, and I'm not your lawyer just because I answer this question or respond to any follow-up comments. If you want to hire me, please contact me. Otherwise, please don't expect a further response. We need an actual written agreement to form an attorney-client relationship. I'm only licensed in CA and you shouldn't rely on this answer, since each state has different laws, each situation is fact specific, and it's impossible to evaluate a legal problem without a comprehensive consultation and review of all the facts and documents at issue.