One Week

The reward that Rin and Erix got after clearing level 7 cave's single Howling Tomb was simple yet impactful.

A scroll of Insight. One could get an insight into anything with this scroll. If one wanted insight into Wind Element, that person could just write [Insight into Wind element] on the scroll and activate it. 

Write it down, and you could get an insight into that one particular thing.

It was a one-time-use item, so Rin and Erix decided to use it once they faced a solid wall during their Howling Tomb challenges.

…

After Erix and Lilith showed the real deal and whipped Maleina and Lynia to their places, Rin and Lily returned shortly.

The level 8 cave's howling tomb was yet again different than level 7. It didn't have one Howling Tomb, but five instead. However, the four out of five tombs were locked. 

Rin didn't attempt to challenge the level 8 cave's 1st howling tomb, as there was much more to it than just a direct challenge.