most royal ships don't have a time assigned, which is why everyone else is saying there is none. right now anthem and harmony are the only ships in the u.s. that pre-assign a time, i think. from your previous post i know you're on anthem, so you *will* be assigned a time.
rcl check-in isn't staggered by status like disney, so if you check in on the first day you can, you stand a good chance of getting the earliest boarding time - the people with higher statuses won't have checked in before you and taken all the early times like on disney. i think the earliest is something like 11 or 11:30, i don't remember exactly.
fwiw, when we went on quantum and anthem out of port liberty, even though we were all assigned times, the port didn't follow them. it was basically a free-for-all at 11:30 or so when they opened the gates. they let on disabled and suite guests, and then everyone else. i believe i've read they are now trying to follow the times, though.
port liberty is not as restrictive as port canaveral about entering the garage. you can park at the port pretty much whenever. we usually try to get to port around 10 or 10:30 to avoid the long line of traffic heading in, hang out in the terminal for a bit, and then board as soon as we can.