Unfortunately, couldn't pin it down. I tinkered with @ajaxProgress and @redirectMessage but had no luck. Additional guidance would be much appreciated.
The timeout for the specific type of AJAX success/redirect messages you're referring to (there are other cases which this doesn't cover) is explicitly set in the JS.
In js/xenforo/min/xenforo.js find:
Code:
XenForo.alert(a._redirectMessage,"",1E3
Replace with:
Code:
XenForo.alert(a._redirectMessage,"",9999
Where 9999 is the number of milliseconds as desired.
Unfortunately, as this is a direct code edit (and not really recommended) any changes you make here would need to be reapplied after every XF upgrade.
Still, some temporary notifications are too long to read in the short while one is accorded. I literally had to take screenshots to get the exact content of those messages.
One possibility is to make this parameter available in ACP for further adjustment as different languages do vary in length and time necessary for reading comprehension.