On average, a doctoral program in marine biology would take 5-7 years beyond the undergraduate degree. Programs vary depending on whether or not admissions is into a master's program to be completed prior to or direct admissions into the doctoral program. Programs also differ in the amount of coursework in relation to research hours.
I'd recommend taking a look at the specific programs that you're interested in to find out the specifics. If you're unfamiliar with the graduate programs out there, a preliminary search on the internet will produce a list to start.
Good luck on your academic endeavors!