I currently have 20 pairs of dress shoes, but only have around 7? pairs of cedar shoetrees.
The 7 workhorse pairs that I rotate for work on a weekly basis are distributed 3 trees that I use in always the last 3 worn shoes. So each shoe gets a good 3 days with trees in them upon usage each.
I use the other 4 pairs distributed amongst my 13 other nicer pairs I wear on more infrequent occasions. And again, the 4 trees are always in the last 4 pairs to be worn. But here in this case, each shoe that was worn may have a tree in them for days or weeks because I dont rotate these shoes that much, if I even rotate them, because I really just pick and choose which shoe to wear out of the 13 depending on the occasion and what I wear.
I know the ideal will be to have 20 pairs for all 20 shoes, but I think it is ok to pull them out after having them inside a pair for a few days.
Remember , the trees are NOT to absorb moisture (cedar is a catalyst to allow faster wicking of moisture, not an absorbing agent. cedar or any material does not INFINITELY absorb moisture, even silica gel must be heated to a certain heating point to get rid of oversaturated moisture before it can be used again), and trees are not to prevent creases.
Wearing shoes will evidently create creases, it is the quality of the leather that determines the degree and the look of the creasing, which in my opinion, adds character and is a part of the beauty of shoes. A straight out of the box non creased shoe looks artificial sort of and I always try them on and crease them initially even though I won't plan on wearing them anytime soon, just because i like the look of a slightly creased shoe, which adds dimension/curves and character/flavor.
What the trees will definitely do is while the shoes are drying and shrinking back into place after wear, the trees will not allow for accelerated fiber malformage or retraction (my own words, i dont know) and will stunt the development of bacteria and mold within where the insole is deep where the ball and toes of the feet reside (that is where most of the deterioration comes from, not the uppers, not soles<can be resoled> but if the insoles and the insides decay, the shoes are dead)/
It is sort of like to prevent tooth decay from the inside of the tooth. you know? it is not about absorbing water or preventing creases, although to some small degree it does this, but cedar is mainly to hold the shape while resting, and to prevent bacteria and mold buildup.
so after a couple of day or few days on a shoe tree, the shoe can exist in your closet without a tree in it fine i believe. until the next wearing and then repeat.