i've always wondered what the burn-in time is before i see a ghost of the timeline in Track Mode sets in.

For me, it was a few months of constant "now and next" display on my Mark 1. But the player still functions great and it's not a big deal, you can't even notice it unless the screen is solid white. Burn-in on VFDs is not a large issue, and it doesn't affect the longevity of the player.