I have a very flat 16-mile commute each way. I have the 16ah 48V battery and a Biktrix Juggernaut Ultra full suspension fat tire bike with the crank gear upgraded from a 44 tooth to a 58 tooth sprocket.
I ride mostly on assist level 4 or 5 and I go fast. Like I average about 20mph and generally hit 30mph at least once during my commute. In the mornings (setting off at 100%) I arrive at work with about 20% charge remaining according to the display. In the evenings (I charge at work so setting off again at 100%) I make it home with about 35% left. The crank gear upgrade definitely sucks more power from the battery (~10% to 15% more). With the OE 44 tooth crank gear, I think I was able to make it home with about 45% remaining. I was never brave enough to try the round trip without charging though.
One evening I forgot to charge my bike, and I left for work in the morning with only 22% charge. I used assist level 1 & 2 and I was able to make it all the way to work (16 miles) with 2% or 3% left. I think with a very moderate assist like 2 or 3 you can easily do 25 - 35 miles and likely even 40+ if you really push it.