well - I'd say you never waste your time when you go fishing :-)
But keeping track records of your catches might help to figure out which days/times/weather/lure combinations work best for your particular location. Days with a good rating are usually a good indication. But you also need to pay attention to the weather conditions. 
Sometimes those days with major or minor times around sunrise or sunset are particularly productive, especially if you're fishing from the shore.

Mostly if you don't catch anything on a good day it means there are no fish around. 
In that case it might help to move on to another fishing spot. They could also be hanging around in a different depth as to where your lures are.