I forgot to answer you question about water retention. When used with the proper medium, they are so well drained that they are almost impossible to overwater. You can water too frequently but if you let them dry out, you can't give them too much water. I like to water by weight. Fill a containers with medium, pick it up and get s feel for the weight. This weight is totally empty of water. Next, completely saturate the container. I actually dunk mine in a tub of rain water and let them absorb it from all sides. When thoroughly soaked I set them on a rack above the tub letting them drain back into the tub to conserve rain water. I actually built a couple watering tables this year to let containers drain back into the tub. As soon as the container finishes draining, pick it up. That is the full weight. I like to water when the container is between 20% and 30% between the empty and full weights. If you see the leaves wilt or droop (depending on tree type) to conserve water, water them immediately. Chestnuts don't like wet feet, so you definitely want to let them dry out. Apples don't seem to be as bothered by wet feet.
If water conservation is your primary concern, consider the roottrapper bags. They work a little differently. They are coated in white PVC and retain water better. Rather than air pruning, the inner fabric traps the small tips of roots which has the same effect as air pruning them. They are reusable to some degree, but not as convenient for removal to plant. To reuse them you staple them along the seam you cut to extract the seedling. They don't last for as many reuses but they are less expensive than the RB2s.
How frequently I have to water my RB2s really depends on a lot of factors. Once a week is an average when they are used indoors in controlled humidity conditions. Variation depends on the stage of growth of the tree and how much water it is using. Outdoors it is much less predictable. Wind, humidity, temperature, and sun, are all factors in addition to the growth stage of the tree.
Thanks,
Jack