I recently bought a used Meta Mint on the For Sale forum, and I'm very dissappointed with its battery life. It takes 2 9V batteries, and with both my Tysonic and my Plainview NiMH batteries (I have a pair of each) the batteries only last about a day or two!!!
I don't keep the amp on all day (only when I listen to music, although I do listen to music constantly). Also, the batteries barely fit inside the Penguin tin, and in fact push against the capacitors in a not to comforting way. Is this normal in Meta Mints?
Assuming the current draw for your MINT is around 15mA or so, a 170mAh battery (most PP3 NiMHs are round about that figure) lasts for 11 hours or so, and even then that would be a full charge/discharge which is practice is unlikely. Somewhere between 10 and 11 hours would be reasonable I would say. You can increase your play time by approx 10x if you are willing to cart AA battery packs around
Someone probably "overbuilt" it a bit, with battery life consuming features for better SQ. If you check out Tangent's Mint page (tangentsoft.net) you might be able to take a look at the configuration and figure out what's going on. Check out the opamp in particular, and if there's a bandwidth setting resistor (R11) for the buffers, and if so what value was used. Ask around in the DIY forum, there might be someone who could tweak it for you for better battery life.