I almost got caught out on PBP in 2015 because we had only around 64h to 1000km. Despite only sleeping for 1h on the third night, I only had about an hour in hand when I arrived at Villaines on the return. So this gives you around 16h for the final ~200km assuming you were right on the limit at Villaines. I think it took me about 6h to get there from Fougeres with three stops (two roadside stalls and a cafe that was open all night, bless them).
I was coming down with a cold during the final day, and somewhat toured back, having a few roadside kips, so it took me about 13h for the final 200km in the end.