Assuming that you had no direct agreement with your roommate's landlord and made no rent payments directly to them, you have not attained the staus of full "tenant'. Rather, in the eyes of the law, you will be viewed as a "subtenent'. This means the your roommate is your landlord. That being the case, if you had no formal written agreement between you, you are correct in assumng that you had nothing more than a month-to-month tenancy. Accordingly, 30 days notice to vacate is sufficient. Additionally, you are not now legally required to provide any security deposit) although you can still be held liable for any damages done by you to the premises).