(Started Oct/Nov. 2011; finished Oct 2014. Efficiency par excellence.)
I've mentioned before about sitting on a sketch or illustration for a few weeks or months to spot all the mistakes. So how about a few years. Which seems to be what it takes for me to gauge not only any potential progress, but to correct whatever bad drawing habits, shape choices, facial geometry issues I tend to fall into. It's one of the reasons why I'm grateful I've kept most of my mistakes from, shall we say, being immortalized.
(Another from the same batch, roughly same time period.)