How often a landlord may raise the rent depends.
If you have a lease for a fixed term (such as 6 months or a year), then the landlord may not raise the rent during that year (unless the lease says otherwise, which they rarely do).
If you have a month-to-month rental agreement (oral or written), the landlord may raise the rent as often as he likes – if he gives you proper notice of the rent increase ahead of time. State law often requires him to give 30 days written notice before the rent increase takes effect.
If you live in a community that has a rent control law, then the landlord cannot raise the rent unless the rent control law permits him to do so. Most rent control laws permit landlords to raise the rent once a year by some percentage set by the city rent control board.