Maybe a client or legal billing timer, or, software would do what you want. A Google search might yield helpful results
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Okay-I've got no idea why this post wasn't 'allowed', but I'll try once more. Maybe there's a 'desk tally' advertiser here whose toes I'm stepping on. Or something. Anyway, here goes...
I use a 'desk tally' http://www.dealextreme.com/details.dx/sku.9515
I just hit the tally once for each side played, and calculate 45 minutes for each two sides. Not down-to-the-minute exact, but close enough. At least for me. With it's '9999' total, you can keep track for over 3700 hours.
I buy one for each of my cartridges, and use stickers on them to mark the correct cart.