There is no limit as such....
Based on my conversation with my attorney on couple of occassions, there is no max limit as such. But only constraint, you should continue to receive the salary mentioned in the Labor certification(LC) till GC is received.
When we were preparing to file my third stage papers, my attorney requested for my W-2 papers and Federal tax returns to cross-check that with my salary in LC.
Hope this info helped you.