be warned; even when you have a legal right to terminate the tenancy, it does not mean the landlord won't sue you. You would have a defense to the suit but none the less, you would have to deal with it.
just to be clear; have you given the landlord a notice, in writing, of the issue?(a) A dwelling shall be deemed untenantable for purposes of Section 1941 if it substantially lacks any of the following affirmative standard characteristics or is a residential unit described in Section 17920.3 or 17920.10 of the Health and Safety Code:
(1) Effective waterproofing and weather protection of roof and exterior walls, including unbroken windows and doors.
(2) Plumbing or gas facilities that conformed to applicable law in effect at the time of installation, maintained in good working order.
(3) A water supply approved under applicable law that is under the control of the tenant, capable of producing hot and cold running water, or a system that is under the control of the landlord, that produces hot and cold running water, furnished to appropriate fixtures, and connected to a sewage disposal system approved under applicable law.
(4) Heating facilities that conformed with applicable law at the time of installation, maintained in good working order.
(5) Electrical lighting, with wiring and electrical equipment that conformed with applicable law at the time of installation, maintained in good working order.
(6) Building, grounds, and ap
If so, how long ago?
so it is a reasonable time. I have not researched it but a bit of common sense would likely play into the determination of what is reasonable and it is likely the colder it gets, the shorter period of time would be required to be considered reasonable.1942. (a) If within a reasonable time after written or oral notice to the landlord or his agent, as defined in subdivision (a) of Section 1962, of dilapidations rendering the premises untenantable which the landlord ought to repair, the landlord neglects to do so, the tenant may repair the same himself where the cost of such repairs does not require an expenditure more than one month's rent of the premises and deduct the expenses of such repairs from the rent when due, or the tenant may vacate the premises, in which case the tenant shall be discharged from further payment of rent, or performance of other conditions as of the date of vacating the premises. This remedy shall not be available to the tenant more than twice in any 12-month period.
and out of curiosity, what is the temp outside, the temp in the main living area that is heated and what are the temps in the rooms without heat

