I could probably write several pages on this, so I'll try and keep it succinct: much depends on what category you file under. Employment-based Green Cards are filed typically under EB1/2 or 3. Make sure you don't let your employer and the lawyer make all your choices. Do some research.

An EB1 filing is the fastest - but you typically need a PhD or equivalent to qualify. Most other filings are in EB2/EB3 which approximate a MS/BS educational equivalent. EB1s could go as 'fast' as 1-2 years wheras EB2/EB3 could take 3-5 years.

These are typical numbers - counting from the day you file. I know folks that got it in just over a year in EB2 and folks that are still waiting 6 years after filing. All the best!