According to the film's original novelisation, the fleet was still in orbit of Sullust at at the point that General Han and his troupe began to circle the bunker before making their ground assault (e.g. about 30 mins before the fleet arrived in orbit of the Death Star).
We're told that hyperspace travel is 'very fast™' and that Sullust is pretty close to Endor but obviously this is a coordinated attack which means that they need to travel at the speed of the slowest ship.
The vast Rebel fleet hung poised in space, ready to strike. It was hundreds of light-years from the Death Star—but in hyperspace, all time was a moment, and the deadliness of an attack was measured not in distance but in precision.
Ships changed in formation from corner to side, creating a faceted diamond shape to the armada—as if, like a cobra, the fleet was spreading its hood.
The calculations required to launch such a meticulously coordinated offensive at lightspeed made it necessary to fix on a stationary point—that is, stationary relative to the point of reentry from hyperspace. The point chosen by the Rebel command was a small, blue planet of the Sullust system. The armada was positioned around it, now, this unblinking cerulean world. It looked like the eye of the serpent.
Lando pulled back the conversion switch and opened up the throttle. Outside the cockpit, the stars began streaking by. The streaks grew brighter, and longer, as the ships of the fleet roared, in large segments, at lightspeed, keeping pace first with the very photons of the radiant stars in the vicinity, and then soaring through the warp into hyperspace itself—and disappearing in the flash of a muon.
The blue crystal planet hovered in space alone, once again; staring, unseeing, into the void.
The strike squad crouched behind a woodsy ridge overlooking the Imperial outpost. Leia viewed the area through a small electronic scanner.
Leia checked the instrument on her wrist. “We’re running out of time. The fleet’s in hyperspace by now.”
Return of the Jedi: Official Junior Novelisation
This ties in with the film's junior novelisation which also has the action in both locations taking place at the same time, narratively speaking.
Wicket and Paploo chattered to each other, then spoke to C-3PO. Leia turned to the golden droid and asked, “What’s he saying?”
C-3PO translated, “He says there’s a secret entrance on the other side of the ridge.”
Paploo knew a shortcut. The Rebels followed him.
In the Sullust system, the Rebel fleet prepared for their flight to the Death Star. Lando Calrissian was in the cockpit of the Millennium Falcon. His copilot was the Sullustan Nien Nunb. Behind them, two Rebel soldiers checked and adjusted the Falcon’s navigational and shield controls.
From the Mon Cal cruiser, Admiral Ackbar said, “All craft, prepare to jump into hyperspace on my mark.”
“All right,” Lando replied. “Stand by.” At Ackbar’s signal, he pulled a lever and the stars suddenly appeared to streak past the cockpit window as the Falcon roared into hyperspace. The Falcon was quickly followed by the single-pilot starfighters. Then Ackbar’s cruiser and the other larger vessels vanished in the same direction, until the entire Rebel armada was en route to the Endor system at faster-than-light speed.