About ⅓ or a third of the way, you can stop at Granville, France. This is a little over 1 hour southeast of Saint Helier.
About ⅔ or two thirds of the way, you can stop at Fougeres, France. This is almost 3 hours southeast of Saint Helier, or a little over 1 hour west of Mayenne.
If you drive straight through without stopping at all, the total distance is 179 km. The distances shown above don't include local roads to get into each town, so if you're planning a long road trip, make sure you factor in traffic and road conditions, rest stop breaks, and interesting detours along the way. If you're in a rush, you can do the trip with zero stops, and your total travel time would be around 3 hours, 60 minutes.