Christina has good advice. Pharmacists are usually happy to answer these kinds of questions if you're concerned about stopping/starting.

When I was ramping up on lithium, my pdoc said that if I showed any signs of toxicity to just stop taking it. She said nothing about not being able to start up again, or that it could never work for me if I stopped and started.

I had a several year period between lithium prescriptions and it was just as effective the second time (but there was a long gap between).

If it were me, I'd increase at the same rate I did the first time around (for me, it was in increments of 300 every two weeks, but it may be different for everyone).

Mostly, though, I would say not to worry about its effectiveness the second time around.