"Is it worth it?" formula

I don't think such a formula can exist. Some games are super short, but really good, so those will naturally have a very low playtime, yet what they offered made the time playing the game great.
It will also depend on your economic situation. Spending 50€ on a 3h game with no replay value might well be justifiable for someone who has a lot of money, but sound like a terrible idea to someone who can only afford a handful of games each year.

WorthIt = -PaidPrice + (PaidPrice / (EstimatedAveragePlaytime) ) * YourPlaytime

If it is positive it was a good deal for you, if it is negative it was a Bad deal.

Optionally add some voodoo and a random multiplication with Pi. Because it looks cool

Actually, some of the best games I've played only have a few hours of gameplay due to being short.

To The Moon gives a giant middle finger to any formula you can make involving playtime.

2 hours long, but SO WORTH IT.

Mostly I look at price I've paid per hour I played. If it's less than I would've spent enjoying my free time doing something else (sitting in a pub?) then it's worth it.

But as been mentioned before - there certainly are games that don't go into this

It varies from person to person but the thing I use it :

1. If game "gives me" at least 1h playtime per a buck/euro spent on it then it was a fair deal.
2. Did I have a good time in it.

Examples:

1. Of Orcs And Men - paid 10euros for it and played for 10 hours - so N.1 is fulfielldd . When comes to N.2 ... well I had a really good time in it even though I doubt I will replay the game so overall it was worth it.
2. Shadow of Mordor - paid 20 euros and got 19 hours - N.1 kinda fulfielled if you add the cracked test run(1h before delete and getting it on steam) . But when comes to N.2 well ... I had to push myself to get the playtime.. halfway through game started to get repetetive and other annoying details . Overall It wasnt really worth it.

Now there are Long and Short games so N.1 wont matters for them so it comes to N.2