What does the lease say? Go with that time frame. Most states require at least 30 days notice, but I send mine 45 days in advance. This allows the ones that are going to move to give proper 30 days notice of intent to move out or allow them time to come into the office and sign.
I also would offer carpet shampooing, gift card (about $25.) or some other promotion for people renewing. It makes renewing leases easier, mainly if you are raising the rent with the renewal. Hannah Carter's suggestion is a good idea.
Per our lease we have to give 45 days notice of any rent increase, so we send out letters about 50 to 60 days before the lease ends. We offer an early bird discount to those that renew early for a 12 month lease.