Is your question: how often Let’s Encrypt will change the value of the _acme-challenge TXT record that must be created for your domain?
If so, the answer is: it depends.
In the best case, a TXT record (or authorization, in other words) will be re-used for up to 30 days, assuming you’re using the same Let’s Encrypt account. But there are a variety of circumstances in which it will change in less than 30 days.
Ultimately, you should not rely on it staying the same. It will definitely change by the time you need to renew your certificate.
Ideally, your Let’s Encrypt client should be automatically handling this for you. Many clients like acme.sh support automatically updating your DNS and creating wildcard certificates, provided that you use a DNS host that is supported: DNS providers who easily integrate with Let's Encrypt DNS validation