Just completed battery test with MixPre-3.
Length: 1 hour 52 minutes until red warning light.
At 1:55 recording was auto-shutdown and got message to turn off recorder.
4 Eneloop rechargeable batteries, fully recharged. About 3 years old, but relatively lightly used.
Mic: AT BP4025 stereo mic with 2 XLR inputs.
While less than the stated length, this is satisfactory for my needs and usage.
Probably newer batteries, or better batteries, would result in a longer runtime. Type of Eneloop: regular (not Pro) at 2,000mAh.