the "real time" gets cut down by various factors, including faster ships and if you have friends in your uPlay list who also fleet. I don't know the exact numbers but I have seen 24 hour missions done in under 5 hours (me or wife runs the app in morning and before bed, tablet is next to bed), I assume because friends on uPlay are also fleeting.
The first thing I did was get 15 full ships though so they are all out sailing, sometimes you'll get 3 or 4 full ship rotations in a single day and even at 1500/ship that adds up really fast. Don't max out your fleet with full 80 galleons, man o wars are super slow so I have a mix of 4-5 frigates (50 slot), 6 70 slot man o wars, and 2-3 80 slot man o wars.
There were two easy ways to get 80 galleons, they are level 49 man o wars and one sits at anchor and guards one of the southern ports so you can just keep attacking and capturing it (and then dying to the second ship that accompanies it) and it'll "reset" that ship while its still in your fleet. Also when you get to level 4 wanted a 80 will come after you. And a ton of 49 level ships sail around the south areas. 675 diamond = 70, 715 diamond = 80 on salvage screen. Send every ship you come across to kenways and you can buy 15 ship slots really fast.
also don't forget to go off and do all your treasure maps, some of the treasure chests drop 4k reales.