I have one ancient plant now in my greenhouse, but I don't know how many time takes to produce around 100 seeds.
It is a geometric progression. Remember also that you can get multiple seeds per fruit you put in a Seed Maker. On average, you get two. So here's an example progression
Week 4: 1 fruit (2 seeds)
Week 5: 1 fruit (2 more seeds)
Week 6: 1 fruit (2 more seeds)
Week 7: 1 fruit (2 more seeds)
Week 8: 3 fruit (6 seeds)
Week 9: 5 fruit (10 seeds)
Week 10: 7 fruit (14 seeds)
Week 11: 9 fruit (18 seeds)
Week 12: 15 fruit, so 30 seeds planted
Week 13: 25 fruit, at this point you're almost done re-planting and just waiting on the rest to finish growing, because that's another 50 seeds
Week 14, 39 fruit
Week 15: 57 fruit
Week 16: 87 fruit
Week 17: All 116 Ancient Fruit is now fully bloomed and you are ready to start processing
That is, of course, assuming a single seed and an average of two seeds per fruit.
The upside is that if you want to plant outside on Spring 1 for a year's crops, you can run a week's worth of fruit through a Seed Maker for a couple hundred fruit.