What if I do not travel US even after my petition expiry date(Sep-2018)?

My H1B case got approved on Oct-2017 and petition expiry date show Sep-2018. I have never traveled USA. my project got changed and i am going for an amendment now. Probably, it may take 3 months for amendment process.

  1. If i go for stamping on July-2018 after amendment, what would be my visa expiry date considering my petition expires on Sept-2018??

  2. If amendment takes long time say after Sept-2018 which is my petition expiry date, what is next? Do i need to apply for extension?

Thanks,

Madhu