There is no single answer to this question—it really depends on how much you are asking of the battery and drive unit during your ride. If you do your entire ride in the most powerful pedal-assist mode (Boost), you’ll demand more energy from the battery than if you are operating the bike in less powerful (Trail or Eco) modes. Shimano states that you can ride 25 miles in the most powerful Boost mode, 40 miles in Trail mode and 55 miles in Eco mode. Those estimates are based on a rider weighing 180 pounds.
Here’s our own experience: In our several years of testing this system, we’ve found that most of our rides involve shifting between the different modes—Boost winds up providing more power than you want on descents, flats or (unless you are completely bonked) even rolling terrain. Accordingly, when we start our rides with a fully-charged battery showing five energy bars on the display, we can routinely finish a 3 to 4-hour ride with 1 or 2 energy bars still showing on the display. If you were riding in strictly Trail or Eco mode, you can complete even longer days in the saddle on a single charge. A rider on a technical ride with a fair amount of climbing that switches between Eco, Trail and Boost as the terrain varies can do a 5 hour ride which is quite the adventure for most riders.