Just had the condenser fan motor replaced on my 550 sub zero refrigerator. I forgot to asked the serviceperson the following questions: Does the fan continously stay on and what is the normal sound of the fan?

It is not true that the condenser fan is always on on a sub zero -- it's controlled by the control board, and will only stay on if the "vacuum condenser" indicator is on. If the "vacuum condenser" indicator is off, the condenser fan should only run when either (or both) of the compressors are running.

The condenser fan makes a very slight humming sound when it runs. You can open the cover on the top of the fridge (it's spring loaded -- pull from the top), and you can see it running when it's on.

Good day,
On models with 2 compressors the condenser fan motor runs 24/7. It must since the compressors run randomly and that motor must be on since it handles both units.

Fan noise is subjective. It itself makes little noise but the blade does. If for example the blade was changed from the nylon style to a metal blade (good idea) then it would be somewhat louder.
If the noise is not objectionable then I would use it as is.