PG seems to be putting it in very concrete terms. For example, he mentions spending only 8 Christmas with a young kid who feels it's "magical". 8, he argues, is an objectively small number for lots of human experiences, like "8 books" or "8 times I dined my favorite meal".
It's just a common saying that's meant to remind us that we have a tendency to spend huge amounts of time and emergy on things that ultimately don't matter, and hopefully sometimes short-circuit the thought process that causes us to do this.