I think it depends on how the bequest was set up originally and who had to manage it. In 1841 Sir Frances Chantrey ( British Sculptor) left his fortune to the Tate Gallery to help them buy new British works of Art. There were some serious arguments, that made it to 'questions in Parliament' when the managers couldn't agree how to spend the money.
However I also know about a small bequest from a businessman in a tiny village in Wales. Alive in the 1930s he gave all the children in the village a Christmas gift and bought the Christmas tree for the school. Obviously the money has dwindled but the Parish council still manage it and now every new child born in the village receives a tiny gift of 50p.
I guess if the managers of a bequest invest it in some way it might last longer, but perhaps not
Karen