Here are some screenshots of the battery detailsFrom BatterymonFrom Thinkpad Power Manager.
I'd say it lasts about 5 hours at most, but I've only used the computer on battery for one cycle. I find it a bit odd that the design capacity is 4070mAh(65.12Wh), which is quite a bit less than 5200mAh. I think I'm going to point it out to the ebay seller and see what they say about it. Aside from the underwhelming capacity, I've never used a laptop battery that stayed so cool before, but that might just be due to how it's mounted on the laptop -- this is the first machine I've used with the battery hanging off the back like this. I think this could result in a measurable difference in lifespan, because Lithium Ion batteries age faster when they're warmer.
My last machines, Dell Inspiron 600m and IBM Thinkpad R40 had batteries under the palmrest, where they got quite warm. I had a second (expansion bay) battery for the 600m, so it would literally run for eight hours at low brightness, I had been hoping this x61 would do the same on a single battery, but it doesn't look like it will unless I can find a better battery.
That looks exactly like mine. i think the capacity may seem different because of the voltage.
so far the battery is doing well (it sat for around 2 months), it says 38 cycles but after charging it a total of 351% (~214.8 Wh) it's still at 38 cycles.
edit: seems to be 350% = 1 "cycle" although i'm testing it again. rolled over to 39 after ~353% finally, which means the battery has 137 cycles in reality, if that's accurate. 6% wear.