I understand this can only be an estimate, and that many things can change. But I will like to get a general idea. Also, assuming that the GC process starts right after the H1B is applied/issued.
I would count at least a year to a year and a half from start to finish: PERM, I-140 & I-485 approval. This is a conservative estimate, making room for many unpredictables or things that could go wrong and/or delay. Hopefully you are not a native Hindustan.
Kindly be advised that the answer above is only general in nature cannot be construed as legal advice, given that not enough facts are known. It is your responsibility to retain a lawyer to analyze the facts specific to your particular situation in order to give you specific advice. Specific answers will require cognizance of all pertinent facts about your case. Any answers offered on Avvo are of a general nature only, and are not meant to create an attorney-client relationship.
Generally, the process can be completed in 12 to 18 months.
Mr. Shusterman's (former INS Trial Attorney, 1976-82) response to your question is general in nature, as not all the facts are known to him. You should retain an attorney experienced in immigration law to review all the facts in your case in order to receive advice specific to your case. Mr. Shusterman's statement above does not create an attorney/client relationship.
Although no advise is a substitute to a personal consultation, assuming you qualify for EB-2 category "Employment Based Second Preference" category, and assuming your are NOT born in India, China, Mexico, and Phillipines, you should be able to get a green card in less than 2 years. My best advise: Consult an attorney with your case specific facts/