How fast can an ejection fraction rate go down in a 92 yr old dad?

Rapidly: If he is experiencing a cardiac event. Please provide more specifics.

Depends: It should stay right where it's at unless there's ongoing or intermittent heart damage (like amyloid disease or heart attacks respectively). 60% is "normal" and "abnormal" doesn't start until <55%. But bear in mind that ef is a fairly inaccurate measure (95% confidence window of +/- 12% by standard noncontrast echocardiography) so don't read to much in a few percent here and there.