I have a green card and have been living in US for about 6 years, I haven't filed for citizenship yet but this year I will be marrying an american citizen, in this case can I become a citizen any faster? or will be it the same process as if I would be a green card holder, which is from 18 month to a year?
You would file an N-400 if you've had a green card 5 years, or 3 years if based on marriage. Assuming you've reached the 5 year mark anyway, you can file an N-400 90 days before the 5 year mark.
I'm not sure why your N-400 would process any more quickly just because you married a USC in the process. I couldn't imagine it taking a year or more.
I attached a guide to naturalization for the N-400.
Also, I attached a link to find the processing time by specifying your field office location.
The answer given here should be considered general in nature and should not be considered legal advice, or that there has been an attorney-client relationship established. It is always suggested to retain the counsel of a lawyer for legal advice specific to your situation.
You can file for your citizenship right away even before marrying a U.S. citizen. The processing times will still be the same but you will get it quicker if you apply right away.