server.lua

-- how much time(minutes) before kick the player
local KICK_TIME = 5
-- timer interval in secondes
local TIMER_INTERVAL = 60

function kick_afk()
    for index, player in ipairs(getElementsByType("player")) do
        if( getPlayerIdleTime(player) > KICK_TIME * 1000 * 60 ) then
            kickPlayer(player, "server", "you have been afk for more than " .. KICK_TIME .. " minutes")
        end
    end
end
setTimer(kick_afk, 1000 * TIMER_INTERVAL * 10, 0)