Don't base anything on the graduation date on your orders. Make no plans right after your graduation. I started in class 19 in Pensacola, and due to several things completely out of my control, I now have to finish up/start over up in Newport.
It varies greatly by class and drill instructor and other factors, but it is highly common for 30%-60% of the people that start off in a class to graduate in a later class. Reasons that you could roll that are out of your control: getting sick (everyone gets sick to a degree... While I was at ocs in Pcola, I had bronchitus, tonsilitus (sp), mono, and a chest cold), injuries (you could sprain an ankle, roll a wrist, or just be injured by someone else. I saw someone get kicked point blank in the eye by a classmate on accident. The DI was dropping the whole class (to pushup position) in a confined hallway. They were too close. Less than a second later he had blood pouring out of his eye. Sprained fingers, mashed knuckles, pinched nerves, rabdo... It can happen to anyone.
Even things you think might be in your control, like the RLP inspection sometimes aren't. RLP can be objective, but it is often highly subjective. My first class had 23 RLP failures (only 19 passed). of those RLP failures, only a few passed the reinspect. The rest all rolled to the next class.
There are many challenging obstacles at OCS that can set one back. The majority of my friends that graduated ocs all spent some amount of time in 'H' class. Some are completely within your control, some you have some degree of control over, and some you have no control over.
If you put out, have a positive attitude, and don't give up, you will make it through eventually. If your stars line up, you'll graduate with the class you started at the seawall with.
When will you have a solid idea of when you are going to graduate? Candio phase (the last 3 weeks of ocs). Why? Because you will have passed all graded evolutions (both academic and physical), and the physical training drops off to a minimal, or at least managable level.
Hope this helps.