if you went to school and during second period you got sent to detention i think you would have to stay there for the remainder of that day... unless a teacher says otherwise or issue you more days in detention
Only the one who put someone in detention here on this site can decide when that detention is over - I think it's usually one of the admins that decide that
Depends on what that user did to get in detention. Conditions and time period may vary according to that. For example, if they were put in detention for repeatedly breaking forum rules and ignoring multiple warnings it may be lifted if they can assure an admin that it won't continue the same behaviour.