Hello,

First of all it depends that for what exam you want to study of 18 hours. It's like you are willing to spend 75 percent of your day while studying. First of all you won't be able to manage the time and stress if you study for almost 18 hours a day. If you do so, you won't have enough time left for your other things like having food and getting enough sleep and lack of sleep can cause various diseases.

12-13 hours a day would be enough. Because then only you would have enough time to sleep. Lack of sleep can cause serious problems and you won't be able to focus on your studies. Do one thing,

1. make a schedule to study,

2. Give more time to difficult Subjects or section (In case you're preparing for some competitive exam). Spend 6-7 hours.

3. Then spend 3-4 hours to study the Subjects or Section you are already good at.

4. And in the remaining time, revise all the sections and Subjects you have Studied in past 10-11 hours.

5. Take proper sleep.

Good Luck.