I would call them back and state that it must have been lost in the mail.
They do have the ability to stop payment on the check and reissue a new one. There is nothing in the rules that states they will wait for the check to be returned.
If your refund check was lost, stolen, destroyed or not received and has not been cashed, we can normally provide a replacement within six to eight weeks.
https://sa.www4.irs.gov/irfof/lang/en/irtrconlinehelp.jsp
Although this is not your situation, even if the check had been cashed by someone else they have a procedure to issue you a new check. (involves another form, etc.)