No. Cincinnatus was not the dictator with the longest tenure, the entire claim to fame for the gens Quinctia was that Cincinnatus was one of (if not the) dictators with the shortest tenures. The dictatorship was an extraordinary (i.e. not elected by the citizen assemblies) magistracy that held imperium for six months--Cincinnatus resigned after several days, having finished his job. Or rather, he resigned from his first dictatorship--Cincinnatus held a second dictatorship in 439, which he also resigned before the expiry of his tenure. The dictatorship lapsed following the Punic Wars--the last dictatorship was Gaius Servilius' in 202--until its revival by Sulla, who held the post (or really a different extraordinary magistracy with the same name) between 82-81. And then of course there's Caesar, whose final magistracy before his murder was the extension of his dictatorship perpetuo--following Caesar's death the office was formally abolished by Antony. I should point out that the office of the dictatorship wasn't really "surrendering power to a dictator." The dictator held extraordinary imperium (although I believe not imperium maius, though I'd have to check--the source of the dictator's extraordinary command was that he did not have a colleague, although there was an attempted bill in 217 to introduce the dictatorship as a collegiate office) but there were limits to it--he could not, for example, institute religious acts without pontifical permission (the final dictatorship before Sulla, Servilius' in 202, had to be granted permission to celebrate a festival to Ceres). He was, however, free from collegiate veto and was immune to provocatio, court appeals of the populace