Your apartment lease will run for a specific amount of time - typically 1 year in Davis, although in most of the country, leases are available for terms of different lengths. There are possibly a few month-to-month leases left in Davis. If you wish to stay in the same apartment after your current term expires, you must renew it.
In most apartment complexes, you should be asked to renew your lease 30-60 days before your lease is set to expire. This provides the apartment complex with sufficient time to find a new renter to take over the apartment if you choose not to renew. In Davis, some apartment complexes ask you to commit to a lease for the next year in January or February - 8 to 9 months before your lease expires, and 3-4 months after you have moved in (given that nearly all leases in Davis start on September 1st and run for 1 year.) Once you sign your lease renewal, you remove much of the incentive for your complex to respond to requests quickly, since you are then a guaranteed resident for nearly two more years. Therefore, it is best to delay renewing your lease until you are absolutely positive that you will not want or need to live in another complex. Also note that if you declare that you will be leaving your complex, your management office may send potential renters for the next year's lease through your apartment to look at it. (Different complexes have different policies about allowing walkthroughs for potential tenants.) The potential loss of privacy and disruption should be taken into account when choosing when to sign a lease renewal.