How long will it take if I received a letter stating my taxes were under review , but talk to an agent an was told to do a amended return will I have to wait for both

What is the reason the IRS agent asked you to amend? Amended returns take 16 weeks or more to process.

My federal withholding had to be corrected 

DO NOT amend the return if the 2020 return  is under review ... the IRS should correct this error themselves but it will take time and a human and right now humans are in short supply.    ONLY amend a return if you get an IRS notice in the mail  explicitly telling you what to do.  

Ok thanks