There are no time limits. The collection agency can start calling you before they send the first letter. They do have to send that notice within 5 days of their first call to you. They call until they get the letter requesting validation. Once validation or what the consider validation is sent the calls begin again. If they don’t validate they sell the debt to some other junk debt buyer and the process starts over.fyi.
The FDCPA is not clear on what validation is. Nor does the ca have only 30 days to validate. There is no time limit for them to validate. Sadly enough once they do it’s all over. You can request additional validation at any time. But this should not be repeated for a extended time.