That's very sneaky question to answer. I rode with the folded atlas on the bars plus garmin ( I like to get the bigger picture on the atlas so that you can get some sense out of the road signs you see, and then zoomed pretty close on the GPS to follow track). With my eyesight and the small font on the Michelin sheets, I would have realistically had to stop each time I make a turn or check something, so maybe 30 times a day, 2 minutes each time? Sounds unbelievable but perhaps 3.5 hours saved! (of course the stop might be needed for something else as well).
Finished day 2 off Col D'Aspin in the dark, hotel was in a different village than I had directions for (note do not believe those blobs with arrows on booking websites!). Took half an hour to find with phone/GPS - without it who knows (another hour)?
Decided to finish route to Cerbere on fourth evening which meant in the dark, my planned route put me a D road that seemed more like a motorway, Garmin let me reroute onto safer smaller roads, when finding an alternative off the Michelin pages would have been much harder (as this was the point when I discover that the last time I saw my head torch was when I put it down for a moment on the sea wall in Hendaye, at dawn on the Atlantic side) - time saved - maybe none as the reroute was longer I think, so we'll call that quits.
So magic Garmin time = 4.5hours -so around 10% of riding time ...wow.