If you have already filed an I-485 application and that application has already been pending for at least 180 days, then you may be able to move your I-140 over to the new employee. You should discuss this with an attorney to see if you are eligible to do this.
If you do not have a pending I-485, then the new company will have to file a new I-140 (and a new PERM application if one is required for your preference category), but you are able to retain the priority date from your previously approved I-140.
There is no deadline to do this, but if there is a chance your old employer will withdraw the I-140, then you want to do it right away, as you can only retain a priority date from an approved I-140.
Ari Sauer
Siskind Susser Law firm
1028 Oakhaven Rd.
Memphis. TN 38119
901-682-6455
[email protected]
www.visalaw.com/ari