About a year.

Mistakenly cancelled my H-1B

Why not address the question to the company's attorney filing the premium processing case?

Unused H1B (I797) . Does it need to be sent for H1B petition ?

Provide all the documents to your attorney without needlessly worrying. He/ she knows how to handle the petition filing.

I want to ask what happens if after a second adjustment of status marriage based interview and you are denied

Depends on the reason for denial and how you wish to proceed.

Can an employer hire me if my temporary green card is expired, and the actual green card is not mailed to me yet?

You can also show them your I-751 filing receipt which extends your stay by 1 year.

Cap exempt H1b

If you have not been subject to the cap in the past, you will have to wait until April 1 to be petition for a non exempt employer.

Working part time on H1B

Company B can file a part-time employment petition in your behalf.

What documentation do I need when entering the US on an H1B from a different company than when I left?

Talk to the company's immigration attorney and seek his advise on travel before you depart.

What will happen to h-1b RFE

1) Employer is required to still respond to the RFE. If not, it will be considered abandoned and denied 2) May be. Answer each question...

It may not be necessary to depart U.S. However a full consultation is required to decide one way or the other.