A learning game of Legion should take about 15 minutes. I learned to play Legion at PAX South right before release and have used a modified version of the way the booth volunteers taught me:
Using the entirety of the core box, place all units within range 3 of each other and their order tokens face down.
Position Luke and Vader so that they are within 2 speed 1 moves of each other.
Place a Stormtrooper and Rebel corp unit so that they need to perform a speed 2 move to achieve range 3 on an enemy. This is more easily accomplished by positioning them behind LOS blocking terrain. Then place their order tokens face up.
Have the person receiving the demo choose a faction.
Because of the way everything is set up, the only available unit they can activate already has a face up order token. Now you can teach them how movement and cohesion works. After performing the full speed 2 move, you can now show them how range is measured from the unit leader and how attacking/defending works as well as cover.
Activate the remaining unit for your side, reinforcing the idea of how moving and attacking works.
Now you're ready for a full round which will explain command cards, the order token pool, and issuing orders. I usually choose Push and issue an order to both Corp units. They will probably choose Ambush and give their order to their commander.
After both sides have finished activating they should have a pretty good understanding of the basics of Legion. At this time ask if they'd like to do a 500-point skirmish game.
One thing I didn't mention is that I don't use suppression or courage during the learning demo. I also briefly explain the various actions they are allowed to take during the 'full round'.