Instances are faster if you rarely die. After level 25ish, figure you need to run each instance 10-15 times, full clear, before moving to the next one. Questing is similar exp but you still have gaps because you skipped collection quests. Plus PvP.... etc.

It will be about 10 - 12 days played to 60, assuming you are semi casually leveling. A highly optimized boxing quest guide would probably be the fastest, but none exists.