How long does the black dwarfs stage last?
A black dwarf is a theoretical object thought to be created when a white dwarf cools down until,it no longer emits light. There are thought to be none in existence as the Universe is too young for white dwarfs to have cooled sufficiently.
If a black dwarf does get created it could last indefinitely. If proton decay does actually happen, then black dwarfs could eventually decay as a result.