Only about an hour actually.

FDR had begged off from a lunch engagement with Elanor and guests and was in the Yellow Oval Room in the residence that afternoon(DC is 5hrs ahead of Hawaii). He used the room as his personal study, kept his numerous maritime and naval prints on display and even his stamp collection up there, and was enjoying lunch with aide and confidant Harry Hopkins.

At about 1:40pm(so 8:40am at Pearl) the phone rings, it was Secretary of the Navy Frank Knox saying they had just received an alert from the Pacific Fleet HQ ashore at Pearl saying "Air Raid Pearl Harbor. This is no drill." He could confirm nothing else but would call back soon with more, but that was the first notice Roosevelt had that the long prologue to America's entry into the war was over. This was then about 50 minutes before it got public as radio programs across the nation began to run reports at about 2:30pm ET as news spread.

While this conversation was going on the attack was still well underway, it had begun about 7:50am local time as units of IJN aircraft split off to target installations across the island. Kaneohe Naval Air Station was actually the first place hit, but others like Wheeler Army Air Field and the Schofield Barracks also got worked over along with Pearl and the immediately surrounding air bases like Ford Island and Hickam Field. At about 8:05am the Arizona was destroyed by a bomb exploding or near her forward magazines. And at 8:40am as the phone rang in the White House the battleship Nevada had raised steam, slipped her moorings, and prevented flooding allowing her to get underway in an attempt to make for open sea in compliance with orders to each ship.

Further confirmation would come a few minutes later when Admiral Harold "Beatty" Stark the Chief of Naval Operations called to FDR. He and Knox had been speaking to Admiral Bloch who commanded the 14th naval District, essentially the shore facilities and local craft in and around Hawaii. Admiral Bloch was giving them a live account just as the 2nd wave of Japanese aircraft appeared over the base. Roosevelt took a similar call right after from Hawaii's governor Joseph Poindexter who in addition to asking permission to declare martial law at one point became frantic and seemed to cut out for a second and after he was back FDR exclaimed to aid "My God, there's another wave of Jap* planes over Hawaii right this minute!"

Roosevelt would spend the rest of the day in the Yellow Oval Room as first his senior military leadership gathered and the full extent of the Japanese attack in Hawaii and elsewhere was filled in, then in succession the full cabinet(8-930pm or so), and leaders from both houses of Congress(10pm-1045) and a stream of aides executing, drafting, or having retroactively signed the orders and proclamations needed to get the nation onto a war footing, along with taking a call from PM Churchill with news of Japanese attacks on British holdings across the Pacific too. Before meeting with his Cabinet he also had a few minutes alone in which he dictated to his secretary Grace Tully his speech for Congress the next day(the famous Day that will live in infamy speech) after typing it up he then made several edits to his liking and would also update it later as more attack targets became known. Tully had also been pulling duty as the main line of communication for the phone. Taking it into the adjoining sitting room off the President's Bedroom to write updates that she received from the Navy Dept and pass them to the President.

Harry Hopkins finally insisted and got Roosevelt to bed at about 1am on December the 8th as he had to be up early to give his speech to Congress for a declaration of war as about 1230 the next day.