I would think that there are different levels of being "put on the shelf". Sometimes a member is punished by temporarily being prohibited to participate in family activities and all contact with other members is severed. This lasts as long as the boss considers that the member may be reinstated. Such punishment can range from a few months to several years.
The worst punishment (if we disregard the punishment of being killed that is) is when a member is totally ousted. Crazy Joey Gallo, for example, became a total outcast and considered as a "persona non grata". I´m not sure when Gallo exactly was put on the shelf, but after the Colombo hit he also had a price on his head.