Once a 605b block is instituted with a CRA, it can last indefinitely as long as you don't have it removed, and more importantly, continue to provide the required documentation to prove identity theft. It doesn't automatically expire unless you fail to respond to requests for information. Whether the lender believes you, or not isn't relevant. It's the CRAs that enact and maintain the block put in place and they're required to notify the lender as well.
You mention police report but don't specify whether that was in person, or not. If not, I'd suggest you try to get an in person police report from a sworn officer as they carry more weight than otherwise. This is something that has caused some people issues when reporting fraud/ID theft.
You'll want to keep monitoring your reports to ensure it doesn't pop back up. You'll also want to keep an eye out for any kind of collection efforts from the lender as once they are notified of a block as per the FCRA 605b they must stop any collection efforts and I believe they also can't sell the debt to a third party for collection either.
