I think it's different for everyone. When I was on Lithium my level was around 1.0, then I found out after being in the ICU for a week after about a month of being horribly sick, shaky, no balance, memory loss and a lot more I had Lithium Toxicity and my levels were 4.6. Had to get dialysis because my kidneys were shutting down and I had a seizure. Only a few brief recollections after being brought to the Emergency room via ambulance, other than that nearly all of the 7 days in the ICU I have no memory of. I was on 1800 mg at the time. Of course I'm sure there are people that do just fine on that dosage, and I'm sure there have been people that have gotten toxic off much lower levels than I had.