That is highly dependent on the exact ESP32 model and current power state. They can use in excess of 500mA with peaks of 800mA, and can easily use >250mA constant at 3.3V. Some draw much more than that.

A board with a few bits and bobs on and a single 18650 cell might only last, say, 8 hours on a charge.

Now, a well optimized board with a low-power ESP32 and proper use of sleep states would make that number go from 8 hours to over a week, but that does take a lot of extra effort and may not be worth it over just slapping on another cell.