In my (limited) experience, Willows tend to be very resilient. I've had newly rooted whips get run over and/or mowed down to the ground and still come back up. On the other hand, I've witnessed old, mature trees get broken at the base in windstorms, and the fallen trunk rooted where it fell while the original stump sent up new shoots.
I would think it would just depend on the health of the tree, and the purpose of the coppice. If I were using it for livestock fodder or mulch I'd be inclined to chop it earlier, while the growth is young & tender. If I was going to use it for posts or hugels I'd prefer it to be more mature/woody.
One more thought- I'd be cautious planting the willows where I someday want to put a different tree, just because they are so resilient. Once they're well established, one would likely need to remove the stump & root system to prevent new sprouts when it's time to plant the new tree. Additionally, I've read accounts of people planting fruit trees near an established willow root system and the fruit tree suffered and/or died due to the willow's excessive hunger & thirst taking up the moisture & nutrients.
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