Scott discovered specific pockets of the Quantum Realm that had him travel in time. Which is what gave him his idea to retrieve the stones and save the people they lost. However Janet either never discovered these pockets, or never stayed in them for long.

I could see the passage of time by different with the different points:

-The place where Scott got stuck in Ant-Man And The Wasp/Endgame, a year feels like an hour

-The place where Hank finds Janet in Ant-Man And The Wasp, with the city in the bubble (a different city than Axia for sure), there the decades pass the same as Earth

-Axia City is different, because Kang AND Darren/MODOK didn't age at all...

@Xandermcc and @SpiderFan2104 are correct.

Janet went to a different 'place' within the quantum realm. When they all go in the Quantumania film, Scott doesn't know anything about where they land while Janet does. Plus, the quantum realm is an unexplored place with many different parts, similar to space. There are many places were us humans would be affected differently in space, same in the quantum realm.

Plus Janet references these different sections as a warning for Scott in Ant-Man 2.

There's layers of the quantum realm with their own laws of space and time, like the dreams within dreams in Inception. When you get smaller than an atom, first there's a weird kaleidoscopic layer, than the dark void layer where Scott went in Ant man 1, then the swirling colorful layer where Scott experienced time dilation, and then the layer where Quantumania takes place, and then deeper layer(s) where Kang might be.

Multiverse plot holes yay 😊

I mean it would be different for Janet compared to Scott

Scott was in an area where time itself was different and didn’t move as it did in the real world

As for Janet she was deeper inside the Quantum Realm because where she ended up she was in like a city with Quantum people and Krylar which is why she grew a little bit older as she was in the Quantum City

"We don't know anything about this place. I mean, not even how time works down here. Have we been gone for seconds, or have we been out for years?"

It's unpredictable. Time is weird and different in the Quantum Realm, and most likely in different parts of it, particularly given the "time vortex" line in Ant-Man and the Wasp.

Sometimes/some parts it's about linear to Earth. Some much faster.