If charges travel very slowly through a metal, why doesn't it take several hours for a light to come on after you flip a switch?

I am not sure if you have stated your question
correctly.

Fact is that electric charge travels through the
metal at same speed as thought any other conductor. This is same as the speed of light -
about 300,000 kilometers per second. And it is because of this that a lamp lights on
almost as soon as the switch is flipped on.

In some other
appliance the action may not be as quick. For example, the coil of an electric heater
may take a few seconds start glowing at its maximum intensity. This is because the
energy generated by the passage of current through the coil is does not generate enough
heat energy to heat the coil to its maximum temperature
instantly.