Cellucotton starts as a cellulose just like regular cotton. It's certainly not rayon that is used in clothing. Been using it for months, JC is good too but it doesn't wick as well as cellucotton and delivers a more muted taste.
CelluCotton also lasts longer between rewicking. It is my preferred wick but I do use both, variety keeps the taste buds awake.
Smacksy, like said it's something you just get to know. There is no amount of time that is universal to all cotton builds. Just saturate, fill the well for the wick to pull up and wait till it looks soaked.
Once running you will taste the flavor leaving and then you have a couple puffs before you get a dry hit. You will get to know the loss of flavor in time and be able to catch it before you dry hit.
Error on the side of keeping it too wet, cant ruin a wick that way.