Put together by XKCD's Randall Munroe, this chart tells you how much time you should spend on making a task more efficient based on how big a saving you can create and how often you have to perform the task over a five-year span. It makes complete sense, and is pretty revealing: if you can slice a minute off a 30 minute task you perform every day, then by all means spend an entire day trying to make that task more efficient.
A lot of the time, though, the savings just aren't worth the effort. Just make sure you don't spend too much time reading the chart—because that's hardly efficient, is it? [XKCD]
How Long Should You Spend on Making Tasks More Efficient?