My research subject got what it was looking for in the 2-10mg range, but like mentioned above, depends on what your looking for. M-Blue has lots of potential uses. When I was going down the rabbit hole I was up to 68 different things that people use methylene blue for, and the large majority of them are off label use in low therapeutic doses. But it's also used up to 2mg per kg or more on certain medial applications.
M blue has some interactions with things that affect serotonin or dopamine and it also may have some interactions with stimulants so please research thoroughly and do not rely on just people telling you what they did.