www.oocities.org/athens/acropolis/2691/fed/1795militiaact.html
wikipedia.org/wiki/Militia_Acts_of_1792
The Militia Acts of 1792 was the origin of the 3 month figure in US law.
"Sec. 2.
And be it further enacted, That whenever the laws of the United States shall be opposed or the execution thereof obstructed, in any state, by combinations too powerful to be suppressed by the ordinary course of judicial proceedings, or by the powers vested in the marshals by this act, the same being notified to the President of the United States, by an associate justice or the district judge, it shall be lawful for the President of the United States to call forth the militia of such state to suppress such combinations, and to cause the laws to be duly executed. And if the militia of a state, where such combinations may happen, shall refuse, or be insufficient to suppress the same, it shall be lawful for the President, if the legislature of the United States be not in session, to call forth and employ such numbers of the militia of any other state or states most convenient thereto, as may be necessary, and the use of militia, so to be called forth, may be continued, if necessary, until the expiration of thirty days after the commencement of the ensuing session."
"Sec. 10.
And be it further enacted, That this act shall continue and be in force, for and during the term of two years, and from thence to the end of the next session of Congress thereafter, and no longer." This act was revised by the Militia Act of 1795 in order to give the president more power over the militia.
The Militia Act of 1795 was passed into law on 28 February 1795.
"SEC. 2. That whenever the laws of the United States shall be opposed, or the execution thereof obstructed, in any state, by combinations too powerful to be suppressed by the ordinary course of judicial proceedings, or by the powers vested in the marshals by this act, it shall be lawful for the President of the United States to call forth the militia of such state, or of any other state or states, as may be necessary to suppress such combinations, and to cause the laws to be duly executed; and the use of militia so to be called forth may be continued, if necessary, until the expiration of thirty days after the commencement of the then next session of Congress.
"SEC. 4. That the militia employed in the service of the United States shall be subject to the same rules and articles of war as the troops of the United States: and that no officer, non-commissioned officer, or private, of the militia, shall be compelled to serve more than three months after his arrival at the place of rendezvous, in any one year, nor more than in due rotation with every other able-bodied man of the same rank in the battalion to which he belongs."
The Militia Act of 1792 was repealed by Section 10 of the Militia Act of 1795.
"Sec. 10. [revised to read:]
And be it further enacted, That the act, entitled "Act to provide for calling forth the militia, to execute the laws of Union, suppress insurrections, and repel invasions," passed the second day of May one thousand seven hundred and ninety-two, shall be, and the same is hereby repealed."