This doesn't work. notify-osd (the daemon that displays notifications" was (and is?) hardcoded to
use its own idea of what the timeout should be.  --expire-time is ignored. :(
    notify-send --expire-time $((1000*60*60*24)) summary "$(seq 1 10)"

I gather notifcation-daemon (and others?) do not have this problem, and it sounds like even notify-osd
may have been fixed upstream.