Flash notices should be short If a notice is longer than about 1.5 lines, it likely too long for an auto-dismiss. In these cases, re-examine the content and determine whether you want to show it modally (e.g. in a dialog which the user manually dismisses) or via screen change (e.g. take the user to a new page containing the information)
It always depends upon the number of words inside the notification. If the words ranges for 5 to 10, 3 seconds is enough and for the words ranges from 10 to 20 words, It will be better to have it visible for 5 seconds. Show activity on this post. Some suggestions for showing 'Temporary notification window' 1.
If the words ranges for 5 to 10, 3 seconds is enough and for the words ranges from 10 to 20 words, It will be better to have it visible for 5 seconds. Show activity on this post.
I recommend that you avoid having timed messages at all because you can't determine how long it takes for someone to not only read the message but also understand it. Recommendation: Include a "Dismiss" button to allow users to close the alert in their own time.