Your question has lot to do with the AC21 final regulations and unfortuantely they are still hanging. However, to shed some light on the topic here are some points:
So far, there is no written rule about when one should chagne the job AFTER gettign the GC, However, an acceptable time frame used to 180 days. With the AC21, it is confirmed that if 485 is not approved within 180 days, one can change job. The dispute among lawyers is on the word 'chage'. Does change mean there was an existing job and it has been changed? If that is the case, argues some, GC is for the future job, how can you tie an exisitng job with the future one? Na that is where the AC21 game begins. It will end only when INS comes out with some clarifications.
Now, in the case of changing job before 180 days expecting the co. will go bankrupt is a little risky. However, if it already has gone bankrupt, it's a differnt case! If you change before 180 days, and your 485 is adjudicated after 180 days, all you can use is AC21. Since it is not clear - it is a chance.
The best action in this scenario is to wait for AC21 clarified and keep finding other job in case of lay-off.
180 days count: It is counted as calendar days. The geneal rule is whenever it is less than a month, the days will be counted as business/working days and if it is a long time frame, they are calendar days.
All the best,