I thought I know enough about the subject, but there's still something worth to consider:
Quote:
Reward Your Player
Keep players interested and engaged with the game experience. For example, in an action game you might offer a new weapon, power-up or enemy every 30 seconds or so for the first few minutes, then every few minutes for the next five to ten minutes, and so on. Or in a story-based game offer new items or character modifications on similar time scales to keep the player invested in the story and the game.
