It's a huge initial endowment for sure. It would be a little more fair to include the other $50 billion the Gates Foundation has already given (ie they're up around $100 billion so far, with perhaps another $250 billion yet to come).

The Gates Foundation has ~$45-$50 billion in assets currently and has given away more than $50 billion thus far (~$23 billion of that giving is from Warren Buffett's contributions; Buffett's donations have to be used rapidly after they're given to the foundation (within the year or such if I recall), that money isn't allowed to pile up). The foundation as it's known today - The Bill and Melinda Gates Foundation - got started with $16 billion in 1999/2000.

Gates + Buffett = $210 billion currently. Due to the Buffett money pouring in, they basically can't give it all away as fast as Bill & Melinda had originally planned, so they're likely going to miss their timeline for giving the Gates fortune away (in their lifetimes was the first goal; that shifted to now being within 20 years after their passing).