Comment 3 for bug 257135

It would be really nice to have command line access to this parameter.

There are nearly as many use-cases for this as there are for using notify-send at all.
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This would enable easily scripted messages to update status quickly. As it is, to get messages at the speed of channel surfing, temperature changes, bandwidth use, etc, etc, etc it's necessary to set an arbitrary, system-wide timeout of 1sec with notifyconf ("Notify-OSD Configuration" utility available from ppa) and set --expire-time < 1000. (This makes a notification which blinks in and out of existence.)

Case in point: I'm using notify-send for some scripts that change tv channels and volume. It would be unnecessarily complicated to write and compile a program to do these things, and since I am using a shell script I can't avoid waiting for one message to disappear before the next appears.

To be precise I want to do this:

notify-send --replaces_id=tvvolume --icon=gtk-info --urgency=critical --expire-time=999 "Volume" "`v4l2-ctl -C volume`

I imagine this command would set the id for the first notification to "tvvolume" and replace that with other notifications that have the same id.... Of course an implementation on par with gnome's volume indicator would be best, but before that I have another long list of feature requests.