8 days, 3 months, 6 months or more? How much time should I ideally spend to cover the SCJP syllabus?

If you don't have a tight schedule, consider covering the syllabus within the shorts possible time. It will be more productive, more efficient and more effective.

The more time you take, you may need to spend more time to revise the previously covered topics. Also consider the time and money you spend just to travel for the classes. More importantly consider the waste of your valuable time. If you follow a course for 3 or more moths just to cover around 50 hours of training need, most of your other time is just wasted.

If you are already engaged with other work (for example you may be a full time student or a working professional), you may not be able to go for a weekday training program. Then you should look at a training program that may run through a lengthy period of time.

Finding a training program that will match with your time availability is your responsibility. Make sure you do so in a manner where you will not be idling excessively. For example, if you spend more than 2 weeks only to receive SCJP qualification but does not do anything else, it is more likely that you are wasting your time.