-file the H1 extention for another one year at a time before your 6-year H1 expires in advance up to 6 months. If you can get your I-140 approved before your h1 expires, you can extend your h1 for another 3 years at a time.
If I get approval of H1B extension but later on labor certificate is denied, which date shall I take as my H1B expiration date?
-Your H1 extension is still OK until it expires again
Here is an artilce for you to do some homework:
http://www.immihelp.com/visas/h1b/h1b-visa-extension-beyond-6-years.html
Thanks a million in advance!