> There was a joke at Princeton back in the 90s that if the board of directors ever allowed the university to spend down the endowment, they could fund ever-increasing tuition for all students for the next thousand years.

Back of envelope: Princeton endowment ~22B. Enrollment ~8200. Current tuition ~$45K/yr. 22 * 10^9/(8200 * 45000) = 59.6.

So in ~60 years, without even adding in inflation or the "ever-increasing tuition" or allowing more students as population increases, they'd empty the endowment.