For most brands, if you leave your electric toothbrush on its charging stand for 24 hours, it will be fully charged - the maximum time a toothbrush takes to fully charge is 22 hours.
If you’re unsure whether your toothbrush needs charging, have a look at the battery icon, typically placed on the front of the brush handle.
If it’s green, it will be fully charged, if it’s flashing green it means that it’s charging, and red, orange or yellow colours mean that it’s low on battery.
Most brands encourage you to leave your toothbrush on the charging stand at all times after you’ve finished brushing to ensure it’s always got plenty of charge, but always read the instructions provided in the box.