When your priority date (the date you file I-140 if without Labor Certification) reachs the then-current visa cutoff date, you may file 485. The processing time at USCIS is projected to be within 6 months by end of FY 2006, but FBI name check could take from a couple of days to a couple of years, depending on your luck.
Put all together, under current situation and projection, your time to GC would range from 6 to 8 years. Any factor that changes in the future could change this time dramatically.