The rule of thumb is that check-in will start somewhere between 10 and 11 but you will not be able to board until around 11 or noon. Cabins are not available until somewhere around 1 p.m.
I would arrive around 11 - 11:30 a.m. and check-in. By the time you go through the check-in process you will "probably" be able to board.
These are all generalities and times can differ due to late docking or other reasons but they are basically the same for most ports.