Message Time-outs
You should avoid having messages time out. However, if you must use time-outs, you should provide an option to disable them in your application.
There are many reasons why a user may not spot a warning that is only displayed for a brief period of time. For example, the user might be using a screen enlarger and may have to reposition the viewport or adjust other attributes to read the text correctly. He or she may take a longer time than normal to type in an answer or take a longer than average time to read and understand the message. The user might even step away from the desk for a moment.
If a message is really important, the best way to make sure it is seen it to display it until the user consciously dismisses it. Even if a message is unimportant, it is disconcerting to have the message disappear before it can be read. If the user does not have time to fully read a message, how can he or she determine whether it is unimportant?