The PH doesn't list a duration for the deafness caused by a Thunderstone. I sent a message to Sage Advice to find out how long it is supposed to last. The following response has only been edited for better formatting. All typos are left in their full glory. The Sage's answers are in bold.

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Date: Tue, 29 May 2001 15:57:00 -0700
From: Skip Williams <thesage@wizards.com>
Subject: Re: Thunderstone deafness duration?
To: Guy Fullerton <guyf@apple.com>

Guy Fullerton wrote:

How long does the deafness caused by a Thunderstone last?

Permanent until cured.

No duration is specified in the item description, so I'd assume it is permanent. This seems awfully powerful for a 30 gp item.

It's not hard to fix.