People do light exercises to stay fit, improve blood circulation and stay healthy in general. Light exercises can include walking, light chores, etc. Basically, anything that does not raise your heart rate too much (not more than 50% of the maximum heart rate) can be termed as light exercise. 

An investigation involving change in the duration of light exercises may include its effect on weight loss, heart rate, stamina, blood circulation, etc. For example, we can study the effect of changing the duration of light exercises (say 30 min, 45 min and 1 hr) on weight loss over a period of 1 month. We can also investigate the effect of different durations of light exercise on stamina, etc.

Thus you need to determine which parameter you would like to investigate and plan accordingly.

Hope this helps.