If you file EB1(a) or EB2-NIW, you can do anytime by yourself.
If you file EB1(b), you need your employer support.
EB1 is very quick, but it is quite difficult now, EB2-NIW takes very long
time to get your GC.
EB2-NIW takes very long term to final receive your green card, for example
if you filed your NIW in 2004, you need to wait 30-40 months to receive GC
if you filed your NIW in 2005, you need to wait 40-50 months to receive GC
if you filed your NIW in 2006, you need to wait 50-60 months to receive GC
if you filed your NIW in 2007, you need to wait 60-70 months to receive GC
The later you filed your I-140, the more time and more money you wasted, the
more difficult you will face. So, file your I-140, the sooner, the better.
Currently EB2 priority date for Chinese is 6/1/2006.