Marathon is a special event which allows to get rewards for coloring 7, 14, and 20 pictures. As soon as a marathon starts, there is a banner at the top of the screen indicating your 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).

 

Once there are no rewards left, 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.