Well, the total flight duration records are probably still held by airships as they moved very slow, however they had superior crew rest facilities and enough crew on them in the first place
Searching through the FAI database i found some pretty impressive records, over
55 hours in a single seat glider plane for example or over
57 hours in a double seat glider plane. If i remember correctly the FAI ceased to accept duration flight record applications in the 50ies as they considered them as too dangerous.