Rebel fleet launches from Sullust, "hundreds of light years"
from Endor, a scene split between scenes in the early morning Endor
time. They arrive during an afternoon.
If this afternoon is the same afternoon and scenes from the movie are all sequential, and Endor's day is of similar length to Earth's, then the Rebel fleet took roughly 6-12 hours to arrive. The distance to Sullust should be between 200 and 1,000 light years, meaning that the total speed is 400-4,000 light years per day, or 150,000-1,500,000c.
If the Rebel fleet's launch from Sullust was shown out of sequence, then we know nothing of the Rebel fleet's speed.
If additional days passed while the fleet is in hyperspace, the speed is substantially less.
The novel also tells us this: That "in hyperspace, all time was a moment." We may speculate that hyperspace involves time dilation effects, making the passage of time on board starships unreliable for measuring speed.
Endor-Sullust is 3.5 million c.
4-6 additional days passed, for a speed of 11,000 to 80,000c.