How often a robot needs charging will depend on the model of robot, its battery capacity, and the rate it uses up charge. That will depend on what activities it is carrying out. Generally walking activities use charge faster than when the robot remains stationary.
Pepper will typically run on a single charge for 10-12 hours and can then be conveniently recharged overnight. Other robots may need recharging every 1-8 hours depending on how they are being used.