There is no expiration date on a Witch Hunt, except when it becomes apparent that it's merely a Witch Hunt -- that no crimes can be discovered attributable to the Witch.
The expiration date for the Mueller investigation would arise when further efforts to find corroborating evidence to that which is already discovered and known do not bear fruit. That point defines the boundaries of the case and the limits to the investigation.
Prudent governance would supposedly discourage Witch Hunts. Knowledge that a crime has been committed and a narrowing of a suspect list -- or expansion of it -- shouldn't have an arbitrary deadline until simple common sense concludes that the investigation has run to the end of its string. And the end of the string is defined by a lack of further corroborating evidence.
The GOP stalwarts don't want to see additional evidence, as it might prove the case they don't want proven. They therefore want an end to the investigation -- if possible before the election. They may well understand the ramifications for propaganda and public opinion if the investigation drags on. But the investigation should drag on to its conclusion.