There is this whole Computer Sciences way of doing that, O-notation etc... but it is largely theoretical and concernes with algorithms, not tasks.

It's not completely theoretical. It's based on finding the worse count of "stuff" done. It doesn't have to relate to computer science. It gets paired up as computer science is heavily process involved. If you think about it, average case, which is theta, is applied in real life all the time, like miles per gallon for a car. big-omega for mpg for an H2, i think is 4mpg. I wouldn't want to imagine what O is for it..