Glad to see you on the forum. I don't have any certification in Java and I don't code on every day basis. I code for leisure and to learn new concepts like lambdas. How much time do you think I should devote to myself certified in Java 8? I know it depends on person to person, but since you know how hard it is any thoughts ?
To prepare for the certification is an effective way to put focus on learning new things. Especially, Java 8 has interesting features - mainly lambda - but also amazing APIs, especially streams and date/time API.
For preparation, if you prepare for 1 hour a day for 10 to 12 weeks, you stand a good chance to pass the exam. It is not to scare you, but the OCP8 exam is reasonably difficult exam, so you need considerable preparation.