How long of a message exactly? How many characters?
The length of time a message stays up is dependent on the "Length" parameter in the config file included (axmessage.cfg). The exact equation is "length parameter * number of characters in message" or 2 seconds, whichever is larger. Looking at my own personal config files, I seem to have found the default of 0.07 too short and changed it to 0.12. Play with it I guess and see what works for you.
EDIT: I also have a slightly untested beta version for... 1.3 I guess. If you guys are interested. It's better at drawing in low fps conditions and things like empty secondary warnings now get displayed.