Depending on which martial art it actually is, a black belt takes between 3 to 10 years to get. It’s possible to progress really quickly in some martial arts while it takes much longer with others.
All black belt practitioners can fight but not to the same extent. If the black belt is earned as a symbol of excellence, it’s fair to say that he or she can really fight. If the black belt is earned for mere competence, then he or she have achieved a certain level of mastery but may or may not ‘really’ fight.