Dear how much notice do I have to give?
There is no set rule. You may have offered a new lease, before the current lease expired, in order to start the new rent on August 1, 2012. And, as August is running down, and if you want your new rent on September 1, offer the new lease now. If your tenant declines the offer, you should still charge her the new rent on September 1. The difference for you is your tenant opts to be a month to month tenant, and that affords you the chance to raise the rent again, if you choose to do so. If the tenant accepts your offer of a new lease, she will assure herself that the rent will not increase again for the term of the lease.
The answer provided to you is in the nature of general information. The general proposition being that you should try to avoid a bad outcome if you can.