Because of the extremely low energy density of batteries compared to liquid fuels and the high up-front cost, the capabilities are inherently more compromised than non-plug hybrids or straight internal combustion vehicles. As a result, I don't think there really is one right answer and a variety possibilities is probably a good thing.

If there is a configuration that does break away from the pack in sales I think it will probably be somewhere in the range used by +Honda Accord with its 13 miles and the +Ford Motor Company C-Max and Fusion at 21 miles.

Unlike the Prius, both the Honda and Ford models have sufficient battery capacity (6.7 and 7.7 kWh respectively) and more importantly electric motor power achieve their ranges in normal driving entirely on electricity while maintaining five-passenger seating and some cargo room.

Reshared post from +Green Car Reports

What is your ideal electric range for a plug-in hybrid or range-extended electric vehicle?