I have no idea on the dosage, but the circulating formate in the body is incorporated in a few hours, tops.
The side effect for me was blurry vision. Which I think was very likely intense healing, since my eyesight has leveled up so much. I used to be myopic, now I can focus my eyes and look sharply at something 300 yards away.. it takes effort because I still need to train my eye muscles, but the nerves and optical stuff is on point now. Which is tremendous.
But still, that's what we should be careful with, not to end up like Odin when he lost an eye, drinking from Mimer's well to learn all the knowledge in the world..
So, begin with low dosage, if your eyesight isn't the slightest bit blurry within a few hours, it should be safe to proceed further.
When I got blurry eyes etc then I was actually breathing in HCOO- and I think it would take a looooot higher dosage to get effects on eyes if you put it further away in the body, like for instance directly on some damaged nerves.