There are 3 diffculties in Adventure, Recruit, Warrior & Legend.
Finishing a part rewards 30, 40 & 50 crystals.
It can take anyware from 3 - 20 minutes, depending on difficulty and which part.

Pay attention to the lady here, she wants to give you crystals.





Missions are worth 50 or 100 crystals.

• Win 3 maps
  
• Play a MOBA map
  
• Play a Capture Point map
  
• Play an ARAM map
  
• Earn 10,000 gold
  
• Destroy 10 structures
  
• Kill 5 dragon

All of them can be earned in PvP, Coop and Single player. Most can be earned in one game if you plan for it.

Playing any of the maps will earn you 3 crystals per minute that the match lasted, with a maxinum of 60 at 20 minutes.

There are some repeatable achievments that can earn you more.

• Victory Medal - 10
  
• Gold Medal - 10
  
• Silver Medal - 7
  
• Bronze Medal - 4
  
• PvP - 10
  
• First Victory of the Day - 50; PvP only

Victory can be earned in all 3 game types, PvP, Coop and Single.
The 3 Medals can only be earned in PvP and Coop.
And the last are for PvP only.

So the fastest way to earn crystals would be to invite 1-4 friends to join you to do some Coop - Mission runs. You pick maps based on what missions the team needs to finish. If a player needs the 10 structures, you let them. If no player needs a specific map, play on ARAM, as that one can be finished the quickest.

More players on the team means you can finish a map quicker, but then you might have to take turns on picking what map to take. If there are only 3 players on the team you're going to at worst get the bronze medal.

For Coop you should be able to get about 10 games an hour.
400-500 from missions (aprox)
100 = 10 * 10 from wins
40 - 100 = Depending on Gold/Silver/Bronze
150 = guess at 50 mins of game time * 3
-----
690 - 850 for an hour of doing something fun.