How many kwh does a light bulb use?

This information is written on the glass of the bulb. Incandescent bulbs are usually sold as 40, 60, 100, and 150 watts. Energy saving bulbs use much less, for instance a 20 watt energy saving bulb will produce about as much light as a 100 watt incandescent bulb.
The watt reading on the bulb is the number of watts used per hour (ie kwh).

Additional Answer

The power of a lightbulb is printed on its glass envelope, and expressed in watts (W). Power describes the rate at which it uses energy. Energy (for electricity bills) is expressed in kilowatt hours (kW.h).

To determine how many kilowatt hours are used by the lamp, you multiply its power rating (expressed in kilowatts) by the time (expressed in hours) it is in use. The answer, incidentally, will be approximate, because the power rating of the lamp only applies at its rated voltage, and the actual voltage may be different from this rated voltage.

Incidentally, it is incorrect to say that the power rating (watts) is 'the number of (kilo)watts used per hour' because that would need to be expressed 'kilowatts per hour', not 'kilowatt hours', and is completely meaningless because power is already a rate.