If you want to add it to your autoexec.cfg:
seta con_notifytime "3"
should do it.
There's also another command for text to stay on screen:
Set number of lines or strings of text that remain on screen in team play mode chat mode (messagemode2).
I have mine set at 3600, that works well.
If you want to add that to your autoexec.cfg too:
seta cg_teamChatTime "3000"