Driving by the direct motorway route it should take about 3 hours 30 minutes.
As is usually the case in France, the fastest route using the Autoroute system is also usually the least interesting. A much more scenic route would take you to by way of Aix-les-Bains and Chambéry to Grenoble (probably by the Autoroute for this part) and then off the Autoroute following part of the Route Napoléon to Gap and Sisteron and then passing north of the Lubéron through Forcalquier and Apt to Avignon. Much more scenic and much slower, but you're not coming to France to see several hundred kilometers of concrete, are you?
IMO it would be worth taking this route and spending a night somewhere to give you a bit of time to explore a few of the places along the way...
I would agree, it's a much more interesting route; we would use the motorway route if wanting to go direct, the other if the journey is part of the holiday.
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