Article I, Section 5 of the US Constitution specifies that "Each House may determine the Rules of its Proceedings." And the Senate has done so vis a vis impeachment.
But back to Article I, Section 3: "When the President of the United States is tried, the Chief Justice shall preside ..."
Under Senate Rule IV for impeachment:
When the President of the United States, or the Vice President of the United States upon whom the powers nd duties of the office of President shall have devolved, shall be impeached, the Chief Justice of the Supreme Court of the Untied States shall preside; and in a case requiring the said Chief Justice to preside, notice shall be given to him by the presiding officer of the Senate of the time and place fixed for the consideration of the articles of impeachment, as aforesaid, with a request to attend; and the said Chief Justice shall preside over the Senate during the consideration of said articles, and upon the trial of the person impeached therein.
Under Senate Rule III:
shall continue in session from day to day, (Sundays excepted) after the trial shall commence, (unless otherwise ordered by the Senate,) until final judgment shall be rendered, and so much longer as may, in its judgment, be needful.
And under Senate Rule XII:
The hour of the day at which the Senate shall sit upon the trial of an impeachment shall be (unless otherwise ordered) twelve o'clock m.' and when the hour for such sitting shall arrive, the presiding officer of the Senate shall so announce; and thereupon the presiding officer upon such trial shall cause proclamation to be made, and the business of the trial shall proceed.
So, what happens if Roberts announces he has a tee time or lunch date that he has no intention of canceling or postponing just because 100 politicians finally roused themselves to put on a show?
The Senate is allowed to make rules for the Senate, but I doubt they're allowed to make rules for the Chief Justice of the Supreme Court of the United States.