For me, it depends heavily on the faction I'm playing. As the Byzantines, I have a large navy from the beginning. First, I can block both the Dardanelles and the Hellespont with them. Second, dromons have higher crew numbers than any other tier 1 ship, which means I don't lose them. They can take on even numbers of pirates or any enemy faction and win, which brings command stars for even more advantage.

As Spain, the only navy I maintain at the beginning is enough to block the Straits of Gibraltar while not being driven out by pirate attacks. I don't really need a navy until I go on crusade or decide to try to take Corsica, Sardinia, or Sicily.

England doesn't really need a big navy unless you're trying to force a channel crossing and the continental enemies have a lot of ships in the area. The HRE usually doesn't need a fleet at all, and I wouldn't bother with one with any of the Muslim factions until later in the game. As Venice, I'll have one fleet for crusade or naval combat, but that's it for a while.