I've heard that you can splash for 20 min at a time before getting logged out as long as you are also being attacked. I'm afking with a chicken attacking me to get past the 20 hour f2p trade restriction, and I've been logged in at least half an hour without touching the client. I'm wondering if maybe it's because I'm not fighting back and that somehow makes the afk timer much longer

If you interact with the client at all in anyway you reset the timer. Even something as subtle as moving the mouse across the screen might be enough to reset the timer. Moving the camera a tiny amount is, even having a conversation with someone.

I don't even mind if AFK activities and monster aggression stopped after 5 minutes, that extra 5 minutes afterward would be fantastic. The number of times I ran back to the computer and calicked anywhere just as my character goes to lobby. An extra 5 minutes gives me plenty of time to do a quick IRL activity, let my dog out, or use the restroom myself without losing a portion of a chat discussion or a training spot to a kind player who doesn't realize I'll brb.