This is a common question in law enforcement: once you discover a facilitator or illegal activity, how long do you monitor it before attacking? You can learn a lot by watching, but eventually you have to Do Something or you become complicit.
Considering the large percentage of people in this space that were using BackPage, I think the answer to your question should have been whenever they had proven so effective at busting the pimps posting on there that usage had gone down significantly. As I said, I’d have automated the process, even the production of warrants to get text messages/emails associated with a given ad. Have software fill in the blanks on a template warrant with all the necessary legal language and details about BackPage, auto-email that template to an on-duty detective and have them review, print, sign, and submit it to a judge. Have the software record all the ads in a database and prioritize cases by the number of ads a given email/phone number appears in.
One agency like the FBI could have created the system, then given it to all local law enforcement agencies across the country for free so that they could carry out the process locally. They could have nabbed thousands of pimps and sex traffickers this way.
It’s a challenging strategic issue.