。
The conversation dragged on until three o'clock.
Literal
Talk [topic-は] three-o'clock [until-まで] sluggishly [adverbial-と] continued.
だらだら is a 擬態語 (state mimetic) describing something continuing in a lazy, aimless way — the conversation went on without direction or energy. The optional と after the mimetic is the adverbial particle that characteristically follows mimetic words. The mildly negative connotation is unmistakable.