From Yosemite Valley or Wawona, it’s about 30 miles and an hour each way. This is a destination to build into the day, not a quick spur between Valley stops. On busy days, visitors report traffic and full parking that can add time; other visitors find parking without much trouble, so leave yourself a cushion rather than counting on a quick turnaround.
Glacier Point Road usually opens from late May into October or November and closes after the first significant snowfall. In winter, the view is still reachable by cross-country ski: about 10.5 miles one way from mid-December through March, when conditions allow.