For day-to-day purposes, if you are used to Fahrenheit but not Celsius or vice versa, and all you want to do is get a rough sense of how warm or cold it is outside without having to do arithmetic involving fractions in your head, then remember that there are two temperatures in Celsius that are roughly the same in Fahrenheit but with their digits transposed: 16° C ~ 61° F, and 28° C ~ 82° F. You can then roughly interpolate/extrapolate by about 2° F for every 1° C.
Bob and Doug Mackenzie thought me to roughly convert C to F by taking the temperature in Celsius, doubling it and then adding 30. It gets you in the ballpark.
"I solemnly swear that I am up to no good."
vs.
"053250411391271"
But to be fair, I never end up with nice sentences. It's more like "Thank you, rainbow. Clock firework" and I imagine myself thanking a rainbow and telling it to "clock firework", whatever that means…
As to how long, I think it could've been a couple of months doing a dozen or so conversions. In total it's a very small investment of time, assuming you space it out and don't cram. It really helps to use the Wikipedia mnemonics (like how 4 is kinda like a mirrored R).
Baker's ratios make my family think I'm a much better baker than I am.
Basic risen bread (a "60% hydration bread" ): 100 parts by weight of flour, 60-70 parts liquid, 3 parts salt, 2 parts yeast. Use grams and scale it up by 5 (500g flour), use water or beer for the liquid, knead, let rise for an hour or so, shape, rest for 30min, then bake at 400F for about an hour or until the inside is around 190-200F, and LET IT COOL to sub-120F before you cut in. Or if you're feeling fancy, use scalded and cooled milk, add 5-10 parts sugar, and swap out 10-20 parts of the liquid for melted but not hot butter - and you get a nice rich bread, half way to a brioche. Or go to 70-75 parts liquid, including some olive oil, and kneed for a long time, and you got a solid pizza dough.
Quick breads: 2 parts flour, 2 parts liquid (including sugar), 1 part beaten egg, 1 part fat (oil or melted butter). This gives you a jumping if point for banana breads, pancakes, muffins, and scones. Add or withhold a little liquid to get the consistency you want for how you're cooking it.
I once chose Paradise By The Dashboard Light for karaoke and that was the only time I've ever done karaoke because I'm still embarrassed ten years later at how awful a choice it was. Great song, terrible for karaoke.
Learn some alphabets of foreign languages. Russian is fun because some of the characters looks like English letters but have completely different sounds. Korean is also cool because it looks crazy complex but it's actually extremely simple.
Not necessarily the part for calculating the day of the week for any arbitrary day centuries ago, that's just a useless party trick, but for the current year so you don't need to pull out your phone to check. Knowing that 1/3 (or 1/4 on a leap year), the last day of February, 3/14, 4/4, 5/9, 6/6, 7/11, 8/8, 9/5, 10/10, 11/7, and 12/12 are all the same day of the week, that this year they're all Tuesdays, and next year they're all Thursdays, is mostly easy to remember and very frequently useful.
A completely random ordering of a deck of cards. You can have a deck pre-stacked in this order, learn some false shuffles, have someone pick a card and place it back anywhere they want without marking its location in any way, and when you inspect the deck you know exactly what their card is. And they'll never guess that the way you did it was memorizing the order of every card in the deck.
I'm sure there are a lot more advanced ways to take advantage of this, just a handy ability to have in your back pocket (literally).
If you're going to memorise a deck of cards, you're better off learning something like the Mnemonica Stack as you can use it as the basis for a whole load of card tricks.
I always just used the knuckle trick for counting. The ones that have 31 days are at the top of the knuckle and the 30 (or 28/9) day months are in between the knuckles.
I have this thing about astronomy. Kind of a perspective thing of our place in the cosmos. I try to remember all the distances of planets from the sun and distances of moons from their planets. Also the diameters of solar objects. There's other factoids I try to remember about neighboring solar systems and galactic bodies. For example I remember the black hole at the center our galaxy is called Sagitarius A and its mass is 4M suns. The black hole at the center Andromeda our closest major galaxy at 2.5M light years is 25M suns. The black hole at the center M87, the closest active galaxy at 50M light years is 4B suns. I didn't look that stuff up so tell me if I didn't get it right.
This is wrong? Taking 20°C as an example. Following this formula gives 48°F when it should be 68. Could you perhaps be supposed to add 32 instead of 12?