I would look at the lithium levels in natural spring waters, and try to mimic the amount one would reasonably get on a daily basis just from eating / drinking out of habit throughout the day. Its probably on the order of 5 mg, but would be spread out during the day.
If ya want my personal opinion, I'd not have a problem with 5 mg a day and would expect it to have noticeable benefits within about two months, or sooner.
You realize of course, that I'm not talking about "mental health" aspects of Li, rather its necessity within various tissue and organs.