Juice in the bottle getting darker is from going up to the wick, then back down into the bottle - so how quickly that occurs depends on a bunch of factors:
- the design and squonk of the atty - does it fill really high then drain back every time, or can you squonk it "just enough" for a few hits, without so much drain-back? Also, does it drain back FAST, or is juice continuing to drain back after getting heated?
- the wicking and juice also can impact this
Usually, I refill my bottle when it's down to like 25% or so. And with most of my setups, It stays pretty similar in color for 6-10 refills, sometimes more.
I've learned that If I "scorch" the wick - where I ALMOST get a dry hit? That often contributes to some darkened juice getting back down into the bottle, and I'll plan to change the wick and rinse/clear the bottle next time it's low...
What do you guys do when the bottle gets low? Do you vape till the end or do you refill? I find it harder to squonk when the bottle gets low and just refill. But I can do this only a few times because the juice will get darker and darker, and eventually I have to discard the really dark juice and start afresh. For me, the darkened juice doesn't taste as well as fresh. Maybe I just have to dry burn/change cotton/change coil more often?