5g/day is the general recommendation and most packaging will come with an appropriately sized scoop, notably this is one of the rare ones where dose doesn’t seem to be adjusted by age or bodyweight. I presume because it’s cheap, well-studied, and there don’t appear to be downsides for overdoing it. They’re testing it at up to and possibly above 25g/day for Alzheimer’s.
Some people recommend a higher “loading” dose for the first two weeks to build up reserves in your body more quickly, but if the goal is to start taking it daily, this is really unnecessary.