When you first start Lithium and anytime you change your dose, you should have your levels tested frequently; every week or every other week. But after being on a stable dose for awhile, every 3 months is "normal."

My level usually ranges 1.0-1.1 and I take 1125 mg extend release per day.
I have had toxic levels before. Some of the symptoms I had were hand tremors, diarrhea, losing balance, and being thirsty. You can get toxic levels if you are dehydrated or sick, so you have to keep your fluids up.

If you are on Lithium long-term, you should also have your kidneys and thyroid tested a couple times a year. Lithium can affect both. I lost thyroid function and have to take thyroid medication now.