THE MAGICAL FORMULA OF PLAYTIME?

This depends on a lot of factors, unfortunately.

- How complex are the battles in your game? The player will take time figuring out what to do in them, but you won't. If the player has a lot of options this could take a pretty good amount of time. The battles (and the menu screens and shops spent prepping for battles) could last anywhere from 20% longer to 500% longer. You might not know how long it will take the player to figure out the right option, but you should know how many options there are.
- Does the player have any long-term choices he has to make, like choosing skills to learn or allocating stat points? Expect the player to take a minute or two each time to analyze this; maybe longer if it's really complex.
- Similarly, the more often you have shops and weapon upgrades (and sphere grid nodes and talent points) the player can choose to purchase, the more time the player will spend in the menu. If these are repetitive (like the crystarium in FF13) then the player will eventually be able to do them as fast as you can, after the first few times. But if each choice is interesting or different, the player will have to spend two or three minutes longer than you looking over the options each time.
- How big is the punishment for a game over? How many game overs do you expect a typical player to get? This should be pretty easy to calculate. If you expect the player to get an average of two game overs per dungeon (one on the boss and one elsewhere), and you have a save point every 10 minutes, the player will lose 10 minutes per dungeon to game overs, and probably another 5 minutes to changing his strategy around afterwards.
- How open is the exploration? A game on rails like FF13 won't take any longer for a novice than for the designer, outside of battle. A game with larger dungeons that can be explored will probably take about 25% longer for a newbie, since they will occasionally need to stop and figure out which way they haven't gone yet. A really open-world game might take the player a hundred hours longer than it takes you.
- Are there puzzles in the dungeons? The amount of time it takes to solve a puzzle can be very hard to predict. I have no real advice here except watch testers try to solve the puzzle. Something you can do instantly and you expect players to solve in 5-10 minutes might take some of them more than an hour.
- Do any parts of the game have time limits? You can cap the expected time for those parts, of course. But make you remember that the player might get a game over from running out of time, and take twice as long as intended.
- If, once a dungeon is complete, you put a sign next to it that says "41/57 treasure chests obtained", I'm going to go back in a second time, because I can't help it. The battle and puzzle times will drop to zero for this second excursion, but the exploration time will be about the same as it was the first time.

So I guess you probably need to figure out how much of your time is spent fighting vs. exploring vs. using shops and menus vs. watching cut scenes, because each of those things is going to have a different multplier.

"Whatever it takes you x2" is probably as accurate of an estimate as you're gonna get without knowing anything about the game in question, in that the actual answer is most likely somewhere between x1.25 and x5, with most games leaning toward the lower range of that.

If I playtest a game I usually take much longer to complete it than other would because I check all the walls and possibilities to make sure I didn't miss any bugs.

A speedrun is different, I've seen people speedrun through a game in 2 hours where normal players need 30 hours. Depends on how open world the game is I guess.