I think that the duration of notifications cannot be easily manipulated at present, at least not without modifying some core tiddlers.
I’d like to request an extension to the tm-notify Widget Message so as to be able to control the duration of the notification, perhaps using an additional (optional) parameter where we could define the duration of the notification in miliseconds?
I have had the same thought in the past, as well as the idea of a notification that explicitly needs to be dismissed by the user. One of the catches with extending the notification time is that it can still be replaced by another notification.
So either we would need to introduce a new alternative mechanism for these use cases that displays elsewhere on screen, or refactor the current notifications so they stack.
Other similar use cases include being able to display a progress bar that stays on screen until the task is complete, something that came up while thinking about what a good UI to show feedback for the FileUploads plugin, or a publishing mechanism, might look like.