If you leave a light bulb on, does it use more or less electricity than if you turn it off and on?

Leaving aside the question of bulb life, when you turn the bulb off and on, you need to expend extra energy to reheat the filament back to white-heat.

A typical 60-watt bulb filament might be about 0.7 m long (it's coiled up) and 0.00003 meters in diameter. The density of tungsten is 19250 kg/m3, so the mass of the filament is about a hundredth of a gram.

Figure the heat capacity is about 130 J/kg/K on average (although it is temperature dependent) and the temperature of the lit filament is about 2800 K. Then the extra energy needed to heat the filament back up is about 3 joules, or 3 watt-seconds.

So the extra amount of energy required to turn the bulb back on is equivalent to the amount of energy the bulb normally consumes in a twentieth of a second.