[thackney]

The fossil fuel period is/will be relatively short

Relative to anything that has meaning in using fuel?

[Ben Ficklin]

The period of time that man has/will use fossil fuel relative to the period of time that modern man has been on the earth, which some say is about 100,000 years.

Or you could say relative to how long fire has be used, which some say is a million years.

You can also ignore the fossil fuel and frame it as how long has man used the steam engine, IC engine, and steam turbine.