First, what exam do you wish to take?
Second, it is to my knowledge that will not be stopped from taking any exam. The only problem that may arise is if you pass it, the approval process. Getting the cert is more than just passing the exam, it is passing the exam, providing proof of work experience, getting a member to vouch for you (all of this through the ISC2 application process) and then getting your application approved by ISC2.
I hope this helps.
It is less about the specific timescale but more on the experience gained and your own assessment of readiness to take an exam. Ask yourself, do I have the basic knowledge and have I experience of applying it?
Good luck in your development.
With regard to ratification of the certification, take the exam, if ready, and accept associate status when you pass. You can upgrade to full professional membership later by submitting future experience. The who point is to unite knowledge and experience, there are no short cuts - nor should there be - to professional recognition.
We all have to jump through hoops in life, they are there (in this case) to confirm status and maintain integrity of the qualification. You and your employers/clients would not value the qualification if this were not so.