Legal Question in Real Estate Law in California

A quick check shows no clearly and directly applicable statutes or case law, so I'd say the first step has to be a careful reading of the terms of your lease with regard to how the CAM (common-area maintenance) charges are to be computed, billed and paid. Offhand, I'd say the time lag is excessive and the landlord should have the proper charges figured and into the billing process much more quickly, but there could be valid exceptions. Further, the legal right to bill old charges is a separate matter from whether the attempt to charge them is commercially outrageous.