The time to prepare for a Salesforce certification depends on the experience of the individual. If you are completely new to Salesforce you have to spend a minimum of 10hrs/week and it takes 6 weeks to be ready for the Salesforce admin certification exam. If you have previous experience, you may complete it in 2-3 weeks at the same pace.
If you are planning to prepare for the Salesforce admin certification exam, I suggest you enroll in this Salesforce training course by Intellipaat.
You can watch this video on Salesforce certification by Intellipaat to understand more about this Salesforce training course: