You don't. Unless that's the kind of thing you're into, I guess.
> Is a portrait that took 50 hours to paint inherently more worthy than a virtually identical one that took 5 hours? Is the slower artist the better artist? Is a Bob Ross 'happy little trees, body of water, mountain in background'-image not artistically valuable because he does it in 20 minutes?
You can't quantify art that way. People work at different speeds. I can appreciate that something took some number of hours without knowing the precise number of hours.
> As for skill: I would argue that a random Banksy takes a lot less skill than the average Artemisia Gentileschi (admit it: you had to look her up). Yet, one is celebrated art, the other is virtually unknown and at best 'one among many baroque northern-Italian painters'.
Maybe, but I'm not qualified to make that comparison. Both are beyond my level of artistic ability (I've never studied art nor practiced much). I don't know why one thing or artist gets more popular than another while another who's just as talented (or maybe even moreso) languishes in obscurity. Skill is a factor, sure, but there's no formula that I'm aware of.
> What sort of people honestly look at a picture and ask 'yes, but how long did it take to make? How long had the artist to be trained for this?'
Maybe some people think of it that way. I don't know. I've never asked those questions about any art I consume. I just think something like, "wow, this looks nice, it must have taken a while" or "what would it take to make something like this, I wonder"