While burn-in is possible with VFDs, I don't think it's very likely. If you're really concerned, you could have the time indicator pop up for 5 or 10 second intervals. Something like this should do it:

<line attr="$(trackelapsed)/$(tracklength)" font="medium" color="bright" bgcolor="transparent" y="26" x="-1" width="0" time="+00:00,10,10"/>

The components of the time= field are:
+00:00 - field should be displayed 0 seconds from the beginning of the track
,10 - the field should be shown for 10 seconds
,10 - the field should show again after being hidden for 10 seconds

Or at least I think that's how it works, it's been awhile since I played with it. But if you're concerned about burn-in, that might be an option. Other option is, of course, to make the text dimmer, with color="medium" or color="dim".