I took off 18 months in my early 40s to study a few subjects that really do not have much economic value.

I would say it was ultimately a mistake. While I stormed out of the gate I got way too comfortable after a few months and ultimately lazy. The mistake was in thinking this wasn't going to happen for me. I would also say the subjective feeling of time was much too fast.

The upside though was that was my retirement basically and the realization that is not what I want from life is quite valuable.