Employer change after i-140 approved with 2nd employer

Following events happened :
I1) started working in US with employer A.
2) i-140 got approved with empl A before completing 6 yrs in US
3) Changed employer to B after 2yrs of approval of i-140 with A.
4) i-140 got approved with employer B recently and kept my priority dates

my Question :
How soon can I change employer from B to C ? Do i still need to let some time pass before I can do that in order to keep my priority date with C?