@Bob39
I had this all answered and when I hit reply my trusty computer crashed and the screen went black.
Fred has summed it up, this is the only action on Airbnb that is not linked to a time limit. But rest assured Bob, Airbnb will stay on his back until he either approves the request or declines it.
May be best to give the guest a nudge through the message stream, but the delay could be linked to the guests payment method! There may have been insufficient funds until the guest topped that account up. Airbnb will not approve an extension until it is paid for.
Cheers.....Rob