Just wandering is there a way to change the timer that kicks people for afk

if you are using 1.7.x + then yes you do /setidletime <1-35000> if you put anything more than 35000 then it causes a overload and players get kicked instantly. the only way to fix it would to go in the console and set the idletime from there.

No, there currently is not.
The reason behind this is that the Aternos owners want to make the queue as short as possible as a convenience to other people trying to get their free server up. There is only a limited amount of servers they can have "I would Imagine." So just think about how if everyone had a server running, not doing anything "Being AFK" everyone else's performance would be affected.

Thanks for understanding!