A marathon is a special event which allows to get prizes for coloring 7, 14, and 20 pictures. When a marathon starts there is a banner displayed at the top of the screen showing progress. 

 

Normally new marathon is launched in a 5 days since one has finished previous marathon. However the system stops offering new marathons if you collect all the marathon rewards in the game (you get a special sticker once the marathon is finished). As soon as there are no rewards, the system won't offer you a marathon anymore. Thus there will be a new marathon once new reward stickers are loaded into the game.