Wikipedia includes a table of this data: http://en.wikipedia.org/wiki/CentOS

  *. RHEL 6.0 to CentOS 6.0: 242 Days
  *. RHEL 6.1 to CentOS 6.1: 204 Days
  *. RHEL 6.2 to CentOS 6.2:  14 Days
  *. RHEL 6.3 to CentOS 6.3:  18 Days
  *. RHEL 6.4 to CentOS 6.4:  15 Days
  *. RHEL 6.5 to CentOS 6.5:  10 Days