Legal Question in Real Estate Law in California

Yes, it's legal if the lease is for six months or longer.

First, I'm going to assume we're talking about a residential property, like a single-family house or an apartment. Second, we have to distinguish between advance payments of rent, on the one hand, and security deposits, on the other, where the limit is the equivalent of two months' rent for an unfurnished place and three months' rent if it is fully furnished.

The two months / three months limits are codified at Civil Code section 1950.5(c) in the first paragraph of that subdivision.

The second paragraph of the same subdivision says "This subdivision does not preclude an advance payment of not less than six months' rent if the term of the lease is six months or longer."