How do I know whether I'm ready to switch to p=reject?

It is recommended to enforce your DMARC policy to p=reject policy after you have completed the initial testing and evaluation of your DMARC implementation while on the Monitoring, p=none stage. This will help ensure that any potential issues with your SPF and DKIM authentication are identified and resolved before you fully enforce the policy. Additionally, it is important to monitor your DMARC reports on an ongoing basis to ensure that your policy is working as intended and to make any necessary adjustments.