There are two main reasons for sanding...

1. to make the surface flat

2. to make the surface smooth

Hardwood plywood is already about as flat as it will ever be; it is also
generally pretty smooth. If it isn't smooth enough for you, you can make it
smooth by sanding the finish, not the wood. If you just gotta sand the wood
I would suggest a grit no coarser than 180. How long to sand? Until your
fingers tell you is is at the smoothness that you want.



You missed a very important reason for sanding, or planing.
For finishing you want fresh wood, not wood that has oxidized. It does
make a difference. The wood should not be exposed for too long, or the
finish MAY be less than it could be.