The CCNA exams do not have prerequisites, so you can take them whenever you want. If you feel you are ready after the course, go take it. Hopefully your instructor will provide you with material that you can use to assess how well you know the material. You should focus on learning as much as you can from the course, asessing what you know by comparing it to the requirements of the INCD1 exam, and then filling in any areas you might be lacking. The exam itself is assessing how well you know the topics at the entry level. Go ahead and review those at Cisco's website so you can already compare what you know and determine what you need to learn at the course. Then after the course, you should be able to determine how soon you need to take the exam.