How long does the AFK detection timer take to kick in?

Most AFK players will receive a message stating that if they do not participate in the next 45 seconds, they will be kicked for being AFK. However, how long does it take before that message appears?

The inactivity detection has been implemented in this alpha patch and hasn't been changed since. The AI control will kick in after 3 minutes of inactivity.

Inactive Player Removal

• Very early implementation for removing inactive players from a game has been added with today’s Tech Alpha patch. Please note that much like Rejoin above, this system is a work in progress, and will likely receive changes in the future. Here’s how it works right now:
  • If a player does not issue a game command for 150 seconds during a match, that player will receive an inactivity warning via chat.
  • If a player does not issue a game command within 30 seconds after receiving a warning, that player will be dropped from the game and returned to the Heroes login screen.
  • Inactive players who are dropped in this way will be prompted to rejoin from the out-of-game menus after logging back in.
  • As with leaving or disconnecting from a game before it is finished, the A.I. will take over the dropped player’s hero until that player rejoins the game. Additionally, that player will earn no experience points, quest credit, or rewards if they do not rejoin the game before it concludes

As the answer posted by Jutschge was 2.5 years old and contains several inaccuracies, I decided to test things out myself (it's not on Google anywhere else either):

• Regardless of whether the player issues a game command or not, as long as the player fulfills the condition for AFK trigger as detailed below:
  (a) not dead, and one/more of the following 3:
  (b1) not receiving experience, or
  (b2) not dealing any damage, or
  (b3) not taking any damage
  He/she will be given an AFK warning and subsequently dropped from the game in 45 seconds.

• The time taken to trigger this warning is at 30 seconds of game start, IF the player does not leave the base area, as shown in the screenshot below, which excludes the 35 seconds of pregame time, where it is impossible to fulfill any of condition (b) anyway.

• However, if the player has left the base area but has fulfilled the AFK trigger conditions, the warning will then trigger at the 150 seconds mark, again excluding the 35 seconds of pregame time, as shown below (screenshot taken 1 second late):

• Inactive players dropped this way will not be able to reconnect (at least in multiplayer AI). Their Hero will be controlled by an AI for the entirety of the game. I tried this with 2 computers / 2 accounts. It will appear as a 'loss' in the match history of the player who was dropped for AFK. No penalties will be given for leaving an AI game.

• Hence, if you're going to AFK AI games with more than 1 account, you should disconnect your internet and rejoin before the game concludes, instead of AFK-ing and getting kicked by the system. Note that you do not have to successfully rejoin before the game ends, but you must start your rejoin timer before the game ends.

Disclaimer: I do this often in 2-player AI and have never been in any trouble for it, but I cannot guarantee that this will be the case forever.