5 hours according to Page 11 of the manual
1. Make sure the appliance is switched off. The battery will not be charged with the on/off switch in the on position. 2. Plug the jack end of the charger (10) into the back of the vacuum, then plug the charger into an outlet as shown. 3.While charging, if the battery is fully discharged, only the bottom led will flash on the charging indicator (9) . As the battery charges the LEDs will illuminate one by one and remain lit until all four will be lit. They will remain lit for 10 seconds when fully charged and then turn OFF. If the battery is not fully discharged when charging begins the LED nearest the fully lit LED on the charging indicator (9) will flash. This indicates that there is a partial charge left in the battery. 4. Leave the appliance plugged in for at least 5 hours to initially charge the battery.
While charging, the charger may get warm, this is perfectly normal and safe. It is safe to leave the appliance connected to the charger indefinitely. The charger automatically reduces power consumption when charging is complete.