It is tough to answer. You should figure out tuition and expenses for 4-5 years, housing for the number of years they plan to stay on campus. Obviously more if you are looking at paying tuition and expenses for grad school. I didn't get good vibes about trying to pay off campus housing out of the 529 - look into those rules.
We have two kids, now 21 and almost 20 - entered Texas state colleges two years apart. We started their 529 when they were born with $7500 each. Contributed a measly amount monthly until they were in high school and decided to quit playing club sports. At that point, we took the club sports money and started catching up with the 529 - about $550 per month. Both had between $30-40k in the 529 when they started. We continued to pay the $550 per month into the 529 for their first year in college. Not knowing how far we could stretch the 529, we had them take out some student loans. I haven't told them we will cover the loans - I like them to have some skin in the game.
Once oldest moved off campus, we started giving her $1100 per month for rent, food, utilities, etc. - hers to budget. This was out of our pocket. Continued to pay the $550 per month for each to the 529. Once I realized she had enough in the 529 to cover the tuition through graduation, we stopped paying $550 into her 529. Once youngest moved off campus, we stopped paying into either 529 and started paying her a monthly amount for rent, food, utilities, etc. She had a little more in her 529 because she stopped playing club soccer her junior year in high school and she had some merit based scholarships. She's planning on PT school, so there will be some for grad school tuition.
Oldest graduates in May from Texas. She will still have > $10k in the 529. You can use $10k to pay student loans and we'll cover what ever is left. Any money remaining in the 529 can be used for grad school or covered into a retirement plan.
Youngest is set to graduate May 2026 from Texas State. Since she should get through in three years and had some scholarship money, she'll have more left in the 529. That will cover some of the grad school and paying the smaller amount of student loans she has.
None of this was part of a grand plan, just me juggling money based upon the given circumstances. I posted all of this to demonstrate that the 529 money can be stretched in various ways and that you should expect to pay some living expenses out-of-pocket. I'm glad its worked out that there will not be too much left in the 529. While giving them a jump start on retirement is nice, I'd rather have the extra money for our retirement. They'll get theirs when we kick the bucket.