Depends on the service center and the type of application. Go to https://egov.immigration.gov/cris/jsps/ptimes.jsp and look for I-485 processing times under your jurisdiction.
GC thru work is 3 years.....GC thru relative is 10+ years.....GC thru lottery is 2 years.....GC thru special ability is 2 years.....GC thru political asylum is 1 year.....There's other ways to get GC, I forgot.
Someone double chek these stats, I'm not sure they are all correct.