GM has known for a long time what model year it will be released and has already considered the Mustang release. But the gen 6 release date has more to do with getting internal resourses available, priority #1 is the Vette. There is also the ATS and other cars that iwll chare the same engines and chasis. One thing on our side for a quick changeover from gen 5 ot gen 6 is Camaro is moving to a new plant so stopping production to changeover is not required.
If you want GM to keep the Camaro unchaged look to the Gen 4 Camaro and the DEATH of the Camaro. If GM doesn't keep it fresh it will die...again.