If you truly want to fund 100% of all their college expenses in a 529, $100k should get your covered assuming they'll be enrolling soon and at a state school.
A couple reference points: I have 1 kid that just finished college and another going into their sophomore year. Both are at state schools in TX.
Oldest received a partial scholarship. Our total cost at the end of 4 years, including tuition, fees, apartment, and some living expenses was $50,000. We had about $60k in a 529, so it covered everything, and we transferred the balance to one of our other kid's 529 plans.
Current student didn't receive any scholarships. Freshman year ran about $12k per semester for 15 credit hours, books, dorm, meal plan, etc. He's moving to an apartment this school year. Tuition, fees, books for the upcoming fall without dorm and meal plan is about $7k. However, our total cost is still about the same per semester since we'll be paying for his apartment. At the end of 4 years, our total will be about $95k. It could have been less, but the apartment he's moving into is $1k/month. (apartment for older kid ran $500/mo. Different schools in different cities with wildly different apartment rates).
Our philosophy is to pay for everything their freshman year. However, once they move into an apartment, our agreement is that I will pay for school and a place to live, but the rest of their living expenses are on them (food, entertainment, gas, etc). They need to learn fiscal responsibility at some point.