What is the optimal balance of time spent on research vs. volunteering/leadership/organizing in grad school and beyond?
Short answer: You should spend "some" time on volunteering and leadership activities.

Unlike an undergraduate student, a graduate student is primarily focused on research, so you don't want to spend "a lot" of time elswhere. Even so, proficiency in other activities is something of a "tiebreaker" against another candidate with comparable research activities that doesn't have "outside" activities.

This is in contrast to undergraduate work, where "leadership" is the "whole game" for some students (for later life)--provided they pass their courses.