Like at what level? Or does it matter? I'm still pretty confused on the process despite reading guides. Edit: I have a lot of extra MP already from equips and a bit of extra base, so I'm already doing that part.
Night Lords only: The second you hit your INT target and if applicable, finish mp washing. NLs get more HP from level up ap so you get 5 washes per level. Every other class: Whenever you want. Just stick 1 ap into HP and you can -mp +hp over and over again with ap resets.