your employer can file green card any time after h1 there is no any time requirement.  Currently it is taking about 7 yrs to get green card according to the current visa bullletin http://travel.state.gov/visa/frvi/bulletin/bulletin_3925.html

They are processing 2001 labor right now. This may change in future. but last year july all visa no. was current so some lucky people get their green card in 6 months even with undergrad degree.

Saan Dai