I took an old bike and a treadmill motor, some old wood and nails and now I have a homemade generator.
This people powered generator produced about 300 watts of power. So if you pedal for one hour you will have 300 Watt Hours of usable energy.
You could run a 1 watt LED light bulb for 300 hours. Or you could run a 100 watt incandescent light bulb for 3 hours. You could run a laptop for about 5 to 6 hours.
Of course this all depends on your batteries and wires. And you must account for any losses in the system. .