Any time it wants to. Not enforcing the law is just another way of say no one is tasked with enforcing the law. And it is the executive branch which does such tasking. And it was intended that way from the beginning. If Congress doesn’t like how the country is being managed, they can fire the President.
To give a serious example: only Congress can declare war, only the President can direct the troops to fight, and he doesn’t need a declaration of war to do so. Congress can control how much money he has available to fight, but can’t tell a single soldier “shoot”.
Legally, the balance of power between the three branches favor’s Congress. Neither the President or any of the Justices can fire a single Congressman, they can’t legally even prevent them from exercising their power.
Police are not even required to protect people: https://law.stackexchange.com/questions/23539/are-police-legally-required-to-stop-a-crime-they-see-being-committed
The idea that the President is legally obligated to enforce the law is just silly - in order for it to mean something, it would require creating law enforcement and prosecutors and judges that are not under Presidential control. It would also pretty quickly end with the President and executive branch totally under the thumb of Congress, as anyone that objected would be arrested and convicted for not giving enough priority to whatever.
Congressional Investigator: Special Agent Jane didn’t respond to a report that someone had violated USC §1011(f) & 36 CFR §261.4(b) (which makes it a crime to say something so annoying to someone that it makes them hit you in a national forest.)
Congressional Prosecutor: On the date in question did you respond to an alleged violation of USC §1011(f) & 36 CFR §261.4(b) in Yellowstone Park?
Jane: No, I was 2500 miles away raiding a terrorist cell that planned to use a dirty bomb to wipe out Miami.
Congressional Judge: Guilty. 20 years to life.
Congressman: when I say wash my car, was my car.
President: Hey, I personally told the director of the FBI to make taking down that cell a top priority after it was reported to me.
Congressman: wash my car...