I'm on PMS-Lithium Carbonate and right now I only take 600mg which is really sub-therapeutic but it works for me. If I take 900mg I start to get the hand jitters and get sluggish, 1200 mg is what I started on and people would really notice my hand shake when I picked up a glass and I felt like sleeping an hour after I woke up and really needed a nap in the afternoon. I would fall asleep on the couch around 7pm and finally go to bed around 8pm.
I have a more anxious irritable bipolar and seroquel XR does a good job for me. I have 50mg pills, I'm on 150mg now but can go up to 300mg if I feel I need it.

I also think not all of us need to be on the same dosage all year round. I know Nov-Feb are key times for depression and I can get a high in the Spring. In the middle of the warm bright summer I usually don't need the same amount medication.