Actually the Constitution says exactly that in the 10th Amendment "The powers not delegated to the United States by the Constitution, nor prohibited by it to the States, are reserved to the States respectively, or to the people."

A deadline obviously exists: the day the electors cast their votes, which the Constitution stipulates to be "the first Monday after the second Wednesday in December", and which this year is December 14th. Other than that, as long as a state properly sends its designated number of electors to vote, how it chooses those electors is nobody's business but the state's alone. If the state is not able to decide, or sends the wrong number of electors, then it creates a problem for the Federal Government, so the Congress can pass a law to clarify what to do in such a case, and since this was the case in 1873, it passed the Electoral Count Act, which essentially sets a deadline for such troublesome states 6 days ahead of the 14th December date [1].

[1] https://crsreports.congress.gov/product/pdf/IF/IF11641