Instead of elapsed time it should be the time to some deadline.
Fascinating how opinions here seem to segretate neatly into two classes:
a. The pragmatic view: The steepness of the curve is imposed by the task. The level of skill to be attained is clearly defined. Learning is learning to do.
b. The philosophical view: The learning process is open ended: learning is learning to know. The steepness of the curve is a (positive) subjective experience, a measure of success.