dl7awl

It's a little hidden. Above the left menu and the Button "New Discussion" there are some small icons near your username (at least on most of the pages). Click on the rightmost symbol (gearwheel) and then select "Edit Profile" and click the menu item "Notification Preferences".

Indeed, I also didn't find any explicit info on that. But why not quickly try it out? I entered 1, and less than 1 minute later the stream stopped, and I got the following message:

So it's clear the timeout value has to be entered in minutes.

BTW, since the actual elapsed time in my case was significantly less than one minute, I suspect that the timeout value is only checked at full minutes and therefore an uncertainty of up to 1 minute can arise. ;-)