I went with 1 day for each arcana check until they succeed
it doesn't say anywhere, so upto you i suppose - it's one of the things that's not given enough detail in this AP - another example is room height - pretty much my whole party is flying atm and only some rooms are given heights so I've had to roll for random heights within limits - also Igramalash's (Book 5) fear aura doesn't say anywhere specifically what it does (the ability in universal monster rules says it could be this way or that way - it should say in the statblock what version it is)