Cap exemption H1B - Time frame for applying

My H1B was from 2008-2011, i was in US for 6.5 months in 2011 and while returning to India i filed for extension and came back. I got petition papers till Jan2014, but i have not gone for stamping.

My question is: Within how much i should be filing for cap exemption? or After jan 2014 can i apply for cap exemption,is there any time within which it should be claimed the unused no. of years in US.