"That's the main road. We'll be there in ten minutes .... Just as well, it's getting light..."
-- George Weasley, airborne over Devon (CS3)
Leaving Privet Drive in Surrey, Fred and George fly the Anglia west to Devon, toward the Burrow. The trip apparently takes several hours, since when they leave it’s nighttime and when they arrive the sun is coming up (which would be just before 6 am at that location and on that date).
They arrive at the Burrow just as it's getting light. On August 4, 1992, the sun rose at 5:45am, so we can assume that they arrived just a little before that time.
Privet Drive is most likely located somewhere in the northern part of Surrey, a little south of the town of Iver. The distance by road from Iver to Ottery St. Mary is 150 miles. If the Anglia travels by air at about the same speed it travels on the ground, it would take about three hours to make that trip following the Muggle roads.