MLF (I use the term conversion as it isn't a fermentation) is used in red wines to convert a harsher malic acid into a smoother lactic acid. In a red wine it is a softer acid that helps in the mouth feel. The Lalvin 71-B yeast has some of this capability naturally, but not enough to tackle something like a red wine. Most wine kits do not include dealing with MLF as you don't know when it is done without either waiting several months or using a chroma photography test kit.
How fast it completes is very pH dependent. At a pH of 3.4 you will likely be done in 31 days, but I would certainly wait it out to 60 days without a test kit. At 3.3pH or under you probably want a test kit as you are likely in the 6 month area for completion; granted we don't usually have mead down that low in pH.
MLF bacteria *can* destroy the mead/wine if you use sorbate before it is complete as they will start to eat that and give off flavors that you do not want... You better be really sure that it is done if you want to use sorbate and back sweeten. My first red wine pyment, I gave the MLF before I used sorbate and back sweetened.
Maylar is correct about that resource. I believe this is the one:
https://gotmead.com/blog/gotmead-live/11-14-17-ryan-carlson-science-fine-tuning-mead/
As far as acid goes, it comes down to using graduated vessels so you know exactly how much you use into a specific volume. You do very small bench trials, say 60ml (~2oz) and weighed acid additions to try and find the sweet spot of the type of acid(s) and how much. Once you know that, you scale it to your remaining batch size. I am also afraid of ruining a batch and I will shoot a bit under the final calculated amount and then work myself up to it.
