A lot depends on what exact bike you have, what motor system you have, what battery capacity you have, your weight, riding style, terrain, etc. Check out Bosch's online distance CALCULATOR for one reference point. Plug in your variables and see what it spits out. I've found it to be pretty accurate.

Also notice the "dual battery" toggle. Some Bosch bikes support dual batteries, which might be just what you need. Or just purchase a second battery for whatever system you end up with.