Princeton's current 2018 size is $25.9B. In 2001 it was $8.4B, so a annual growth rate of over 6% per year.

So Princeton would have to spend 1.6% of their endowment every year to offer free tuition to their students. Assuming their rate of return on the endowment (including alumni contributions) can grow faster than the cost of tuition growth, they can provide free tuition for their students indefinitely.