How many miles is an unmarked vehicle allowed to follow closely behind a car on the interstate in order to find some reason or traffic violation (e.g. illegal lane change)? Isn't there some case law that states an unmarked vehicle can't follow for too long because some traffic infraction is bound to occur?
Until it runs out of gas.
With the caveat that I can't prove a negative: No, there is no such statute or case law restricting how long a police or law enforcement vehicle can follow someone on a road.
However, law enforcement officers can be subject to investigation and sanction under "stalking" or "harassment" laws, which typically require a pattern of documented misbehavior in the absence of good cause for said behavior.