MSFT is up 18x during that time and the S&P 500 is up 5x during that time. His investments are some mixture of MSFT and other things, so we might say he would have been up around 10x if he'd given no money away.
Since his net worth is only up 3x, that means he gave away about 70% of his wealth.
First giving away X% and then getting a Y% return on investment has exactly the same effect on his wealth as first getting a Y% return and then giving away X%.
So to determine X, we can just ask how much money he’d have now had he not given any away, and it looks like he has about 70% less than that.
However long he took, he has ~70% less wealth than he would have had if he didn't do it. If it took him longer, that only means he gave the wealth after it had more time to appreciate.
Since his net worth is only up 3x, that means he gave away about 70% of his wealth.