I think that depends on the nature of the museum and the collections.  A very
 small local history museum or historic house usually has a very high % on
 display.  In my experience 75% to 80%.  Larger more general history museums 5
 to 10% by number- more by "volume".  Such as most of the larger objects
 (carriages, furniture and the like) but maybe 1%, or less, of the number of
 textile items.