At present, the American president is probably one of the busiest people on the planet with intensive time management to fit in all the duties and commitments every day. Yes, in HW Brands' biography of Grant, there is this portion where Grant says that the President is only truly busy when Congress is in session and the rest of the year is kind of empty, barring emergencies of course. And so he would often leave the capital after Congressional sessions were over.

So my question is, approximately when, from Grant's presidency in the 1870s to American presidents becoming "leaders of the free world" in the 1950s, did the presidency transition into being a very busy job?

My own guess would be FDR's presidency when the New Deal vastly increased the duties of the executive. But I would like an expert opinion.