[mst]

Drawing over permanent marker with a white board marker does work provided the permanent marker hasn't dried yet.

Or, at least, I have done this successfully before now - so my current working assumption is "people are waiting too long after which it doesn't work", but since I like my white boards I'm unwilling to run the necessary experiments to test this.

[tmm]

I don't think time has anything to do with it. Rather it's the solvent in the dry erase marker. I used to use dry erase markers to remove extension numbers from the little tabs on 66 blocks back in the days before IP phones. Sometimes those numbers had been there for 15+ years.

So, my advice would be: if one brand of dry erase marker doesn't remove your permanent marker mark, try a different brand. I think I used Expo.