Flexible session time in AFK Arena:

AFK Arena is a mobile free-to-play game published by Lilith Games. AFK arena is an RPG with idle mechanics. The game is actually celebrating its first birthday with over 40 million downloads.

I analyze but rarely commit to free-to-play mobile games. I easily get bored of the daily repetitive tasks, but it's been almost one year and I keep going on AFK arena.
In this paper, I will take a closer look to AFK Arena's subtle mechanics that intend to reduce long-term boredom. Since, I'm going to focus on a micro perspective, if you're looking for an awesome overview of the whole game, I highly recommend the Deconstructor of Fun's article.

Flexible daily quests: Creating your own routine

In order to urge the player to come back every day, the game sets up a special daily quest system.
The player can accomplish different tasks related to the game's core loop (fight & upgrade) to earn 10 or 20 points:

Tasks directly related to the player's progression are more rewarded (20 points) than indirect tasks (10 points). The player gets a reward every 20 points until he reaches 100 points and obtain a last reward. The sum of every task is superior to 100. This way, the player is able to organize his daily routine and thus slightly reduce the boredom caused by the daily repetition.
This system also give a completion feeling and gently tells the player that he's done the daily job.

AFK Arena restrains the player's progression using high difficulty peaks. Sometimes, the peak is so high that players could just upgrade and leave the game. This system ensures the player is circulating well throughout the core loop, it encourages the player to regularly check his progression and thus be exposed to paywalls.

Game session length variation

In the campaign mod, the player progresses throughout chapters. Those chapters are divided in levels. Every 5 level, the player must defeat a tough boss. The boss-fight is the high difficulty peak mentioned earlier. Sometimes, the player can beat 30 levels after being stuck 1 week. There aren't anticipated limits to the player's progression like "energy points". The player can try as much as he wants to. On the opposite, energy points abruptly stop the player in his progression and the player always knows where exactly he will be stopped. The AFK Arena design is less frustrating because it seems related to the player's skills or convenience.

Moreover, there are many different game mods (Arcane labyrinth, Peaks of time, King's tower) without time limitation so the player can complete them whenever he feels the need to. Once again, it is up to the player's convenience. AFK Arena can provide daily five-minutes sessions and one-hour sessions as well.

Idle Chest mechanic:

The session schedule is centered on the collect mechanic. On the main screen, a chest fills up with resources through time. At any time, the player can collect all of the accumulated resources by taping on the chest and reset the timer. When the timer reaches twelve hours, the chest stops filling until the player taps on it. This mechanic urges the player to have at least two session in a day.

First, the player will always start a session with new possibilities due to the amount of resources collected. To ensure that, the chest is on the main screen with appealing visual effects.
Second, twelve hours-separated sessions aim for two moments of free time in a busy day.
For example, a first game session at 9:00 AM in the subway and a second one before going to bed at 9:00 PM. Depending his engagement and convenience, the player can empty the chest two times a day and begin his sessions with the biggest amount of resources or he can empty the chest multiple times a day to optimize the collected resources.

Bounty quests: A scalable retention mechanic

The bounty quests are another idle mechanic. Everyday, the player can fulfill quests by dispatching heroes (those heroes are still available in other play mods) to obtain rewards.
Unlike the chest mechanic, the player can only collect the resources when the timer reaches 0.
According to the quests' rarity, the reward, the duration and the required heroes are different.
The quests' duration never exceeds 12 hours. In this way, it doesn't overlap with the second session of the day. It becomes a short-term retention mechanic that urges the player to start a second session in a day. However, those rewards can be collected tomorrow or at anytime and thus become another reason to play later. The bounty quest is a scalable retention mechanic and a smart use of the idle-game side.

Conclusion

Because of those mechanics the daily sessions are always convenient, positive and they can satisfy different player's needs from a short to a long game session. To design how many sessions the player should have per day and how do they end, is a good way to ensure a qualitative experience through time. In the end, it's always at the players' will and this is the game retention strength: Flexibility over control.