Near the end of the first cruise, you will get a letter with instructions on the disembarkation process - it varies cruise to cruise whether they have you meet somewhere and escort you off or tell you what time to be off the ship and you get off by yourself. If you are not escorted, wait until the ship is just about empty before leaving -- it's more comfortable on the ship than the terminal and you won't get back on any sooner. After you go through customs, you will go back upstairs to the check in area (in Port Canaveral you don't leave the terminal).
Again, how long before you can reboard varies from cruise to cruise. They have to close out the first cruise -- make sure everyone is off and all onboard accounts settled. Then they can open up the second cruise for check-in. Then they have you wait until the onboard gives the okay to board b2b, vendors, performers, etc. We have been back onboard within half an hour and there were times we were on board just about 10 minutes before new passengers started boarding so it has really varied.
Once you are back on board, if you can go to your cabin depends on how early you get back on board. Once they rope off the area access so new guests can't get to their cabins, they request that b2b'ers are either in their cabin or out of their cabin until all cabins are ready (so new guests don't see people going around the rope and go around it too). We used to always do laundry on turn-around day (started the washer before breakfast, moved it to the dryer before disembarking -- with a note letting them know we'd be back and hadn't forgotten our things, and then got it when we came back on board). The last couple of times we haven't done it this way because we have gotten back on the ship just before new passengers and while we probably could have still gone to get our laundry, didn't want the hassle so we did laundry late on the last night -- when everyone has already packed so the machines were not in demand.
