I used to do this conversion when I was contracting at hourly rates and my time was the bottleneck: should I fix the toilet myself or call a plumber? It no longer makes sense on a 9-5 job. The fungibility options (have I used this term right?) are different.
Another gem from xkcd: Is It Worth the Time? It visually answers the question:
Of course, he's not the first person to ask this question, but the visual is handy. Note that the times are calculated assuming you'll save the time over five years.
For example, I've been pondering how to shorten my showers. If I can shave off 1 minute daily, I should be willing to invest up to (but no more than) an entire day to do it. If I think I can shave off five minutes preparing my breakfast, I should be willing to spend up to six days attempting to do so.