As Plank says use a hours run meter.

A long time ago I made up a box with x2 hour run meters in. One records the 'Total time' the other records 'running time'.

You record the dials at the beginning, and at the end....you do not have to worry about an exact number of hours, days, weeks etc as you have the 'total' hour meter to give you that.

Simple maths then gives you % figure or run time / 24hrs.

Keep it Simple!