That would depend on the shape and size of the fermenter and how well the air is circulated, etc. That being said, if you can keep notes on how long it takes for your system, you’ll know the next time as the cooling time for your equipment should be consistent.
Well I can’t speak for BF, but not everyone will use a corny keg. So that doesn’t simplify. Plus you still have to consider whether it’s a freezer or a fridge (cooling times may differ) and is the air moving, how much space... etc etc. As I mentioned before, it’ll likely be more accurate with your own measurements.Corny kegs are pretty similar in size and construction right?
Also I am sure that there are only a handful of profiles for circulation that could be used. Wouldn't need to be perfect but surely this is not an impossible calculator.
Different size cornies as well.
Dimensions and cooling capacity of fridge would also be variables.
Curious, why do you want to calculate how long this takes?
In want to know how long I need to leave a keg in my fridge before I add geletin - without pulling a glass or two to check temps.
With all the variables described above, I believe you will be better off doing some equipment testing with temperature checks periodically until you reach your desired temps. Document them and that’ll be more accurate than a “generic” type calculator that would be difficult to make accurate for everyone.