I just found a neat feature. /played which is a chat command that displays the time played for the current character since creation and in the current level.
Update: See below.
Since I reupped in May, I’ve been leveling some toons pretty efficiently. I typically used dungeons to level 60, then quests through Outland, Northrend, Vashj’ir and Pandaria. Some dungeons were played after level 60, but primarily quests were used to level. At this point, they continued with Quests and Treasure Hunting through Draenor. Treasure hunting gives about 1.5 times the XP of a quest in the area, so using a Treasure Hunting AddOn can be a very efficient way to level, especially if you can fly through Draenor.
The goal is to get to 101 before going to Legion. All the characters have 4 Heirloom armor pieces, an Heirloom weapon and a necklace. At level 101, there is a jump in Heirloom item level, which will help a lot when starting Legion. Here are the play times for some of these characters leveled this way to about 100:
Before taking these characters to Draenor, I took a Demon Hunter through the DH starting area, and then directly to Draenor to most efficiently get Draenor Pathfinder. So all other characters could fly in Draenor. As these characters reached Level Cap of Outland, and Northrend they moved on to the next area. Before moving to Draenor, some of the classes were taken to 91 to boost the Heirloom Item Level, but in general this is not that necessary. The Pandaria and Draenor starting areas are quite friendly and reasonable even with green level gear.