If you have prior experience on Junos then you can give it a try else, it’s not that easy that you can pass the exam in one week.
You need to understand Junos Basic feature (firewall filters, protocol, policies) need lot of hands on to understand the technology.
I don’t suggest any timeline to prepare for JNCIA, you can give exam anytime if you understand well the Junos features (required for JNCIA), have quite a good hands on and if you are confident.