When the problem was detected on the RV-12 prototype it had over 1000 flight hrs.
Considering the hard use it endures (large number of different pilots and used for transition training) it is reasonable to expect that other RV-12's would have to acquire at least a similar amount of flight time before seeing a problem.
As far as I am aware, the only other installations of the doubler kit have been pre-emptive on flying RV-12's, or during construction of the wings on newer builds.