Can I work for new employer when I-140 is approved but H1B extension with current employer is underway?

USA H1B Visa
#1

How soon after I-140 approval with current employer can I switch to a new employer? Should I wait until the H1B extension with current employer is approved? Or can the new employer file for H1B as transfer with the I-I40 from current employer?

#2

180 days from the date of approval of I-140