One day. That was how long Neve was willing to spend getting her Healing Magic skill up a few levels.
She was only willing to do so due to the discovery they'd made that this floor, it would seem, was going to be composed of mostly undead-type enemies. If that held true, then Neve was confident she could make it through to the 6th Floor in no time at all.
Plus, unfortunately, it was simply true that at some point, Erin's level would need to be higher, given that Neve assumed Thomas was either caught up already or close to catching up.
So, she walked up and burned herself over and over again, taking herself as close to death as she could before healing herself back up.
She did this without wearing anything, with nothing on her but the chip on her shoulder. Using the Angel Set would empower her healing and, thus, reduce the amount of training she actually did.