That depends on the amperage of the charging system and the size of the battery. Keep in mind that most of the time, the battery will not be empty when you plug in, thus reducing charging time.
To recharge a completely empty car battery from an ordinary 120-volt socket, the Chevy Volt plug-in hybrid would need 10 hours and the Nissan Leaf EV would need 20 hours.
Using a faster 240-volt outlet and a charging station, the Volt recharges in about 4 hours and the Leaf in 8 hours. See our Accessory Tracker page for comparisoins of charging stations.
Some states are beginning to install fast-charging stations along highways that can recharge a car to 80% of battery capacity in less than 30 minutes.