It's a setting in the Admin Control Panel, there isn't a User setting to change this. I'm guessing it's a 45 to 60 minute timeout?
If you don't intend to read all new posts, don't visit the forums until you're ready to sift through everything.
Another option is not to log-in until you've read everything, and have decided to which thread(s) you will submit comment(s).
After you have logged-in and submitted comments, and have logged-out, and the screen has returned to the RR forums, then click on "Today's Posts". This will set a cookie which will "start the timer", so that the next time you visit, the "new" messages that have been updated since you logged-out will be Yellow. If you don't click "Today's Posts" (or something similar), then the cookie does not get created, and the next time you visit the Forums, none of the threads will be Yellow.
Hope this helps,