Reg- H1B CAP exemption

Hi, I had H1B Visa from oct 2012 - oct 2014 and was in US from Jan 2013 - Feb 2014 after that i was out of US. After the expiary of this Visa, i have applied H1b under cap exemption and got visa stamped from Nov 2015 - Dec 2016. If this visa is not used then untill which year i can apply new H1B under CAP exemption. Please clarify

Rule of thumb is that you are cap-exempt for 6 years. In this case, you would be cap-exempt until 2021 (6 years from 2015).

However, there have been recent experience where USCIS has approved it even after that 6 year clock has elapsed.

Thanks for your reply on this.