Really can't say what is the best way to know when to write the exam. Just when not to write it.
First do read one or more books on the exam and try their practice questions.
Second is look up for online videos on Cloud Computing and associated areas covered by the exam. For this use other Certifications as well as general coverage of the knowledge areas.
Third is you are not ready when you get 100% on just 30 to 125 questions. You need more that that number and best to find different question sets from different vendors.
From my experience writing the CISSP and passing on my first attempt in 2006 you need to know more then the knowledge. Questions will cover:
1. Knowledge, 2. Ethics, 3. Scenarios, 4. Think like a Manager/Business Leader
Best time is when you are just tired of doing the questions, have had a month off from studying and when you return to refresh you are getting around 90% or higher when doing 125 question tests in 1.5 hours or less. That said you want to practice reading the entire question and all the answers given. Reason for 90%+ is you want to get about 12 or less wrong so of the 100 marked questions you will be at 88% correct if you get only 12 wrong of the marked questions. This said 88% is not guaranteed since you need to show competence in all six domains. Either way what ever the number of test questions which are unmarked for the final passing score you want to get more then half of them correct in the practice tests.
I am hoping this is a wining strategy and works for me when I challenge the Certification.
I used the official study guide and the official practice questions book.
As I'd already passed CISSP that helped a lot.
Not gonna lie, it was a tough old exam, and for sure some of the questions I got weren't topics covered in the two books. Having said that, I understand that you do get 25 'beta' questions, so that could explain that.
The two official books as above are a great place to start, but absolutely make all efforts to study stuff they call out in the book such as OWASP, CSA CCM, NIST and ISO documentation, etc.
The timing of the exam is very comfortable, I finished in 80 mins, so could have theoretically taken twice as long on each question.
My one bit of advice is when you aren't sure of an answer - go with your gut instinct, it's usually correct.
Good luck, it's a great sense of achievement when you pass 😀