If you are docked you can come and go as you please. If the ship is anchored and there are tenders running, they usually run about once every 30 minutes.
Just be sure to be back onboard at least 30 minutes before sailing time. When you leave the ship check the "ship time clock". Ignore any clocks you see ashore!
As AKcruiser said, if you're not in a hurry let everyone else get off first! Eating breakfast or just relaxing is much better than standing in line.