On electric bikes with a twist and go, the bike will be able to take you along entirely under its own power, up to 15.5 Mph. This decreases the range of the electric bike as it asks more of the motor and battery. It would also struggle on steeper slopes - Even the minimum of pedalling is more significant than you may realise!
The electric bikes are really intended to provide electric assistance to your pedalling as and when you feel is convenient, rather than take over entirely. However there are exceptions to the rule in high quality electricbikes like the EZ Bike, which is built to be mostly throttle controlled and doesn't offer pedal assistance. Without pedalling, a typical range will be around 15 miles, but this can be increased with larger or additional batteries available from some manufacturers.
More details about high quality electricbikes via www.ezbike.cn