Board Thread:Game Discussion/@comment-26715739-20150820121023/@comment-26715739-20150910061544

They're intending a 6 week cycle as per usual:

http://www.usatoday.com/story/sports/nascar/2015/08/31/ea-real-racing-3-app-nascar-sprint-cup/71445666/

"Though the NASCAR update will live for only a short time, any cars unlocked by users in the six-week period can be used for the rest of the game."