There is no set time. Parents can help with schedules, deadlines, scholarship searches, college searches, college visits, and creating a college list. Parents should also help students stay on top of grades, register for standardized tests, and have a conversation with the high school counselor regarding college preparation.
As you can see, if you do all of these things, it's a time-consuming process. But it's impossible to put a time value on it. You can do as much or as little as you like to help your child. It's entirely up to you.
If you have additional questions, I will be happy to answer them.