my answer to the Bonsai Talk thread:
Paul,
your interesting remarks explain why re potting and root cutting is necessary as soon as the roots at the bottom of the pot become too dense. How soon this happens depends on many factors. The main factors are plant species, age of plant and watering and feeding scheme.
A root ball that it too dense is detrimental, regardless of what substrate or soil you are using.
I believe in aggressive watering and feeding. Therefore my trees often get root bound pretty fast. I re pot old trident maples and Japanese maples every two years. Most old deciduous bonsai have to be re potted every three years. Conifers are not so problematic, especially very old collected specimens. They usually have only few roots and it takes very long until the are root bound. I leave very old collected conifers in the same pot for five to ten years and longer.
But make sure you know what you are doing! The substrate must be very well draining and aerating. Often collected trees still have the debris of their natural habitat around the rot ball. This debris decays in a surrounding which is moist and warm. This organic matter uses up oxygen and creates very fine particles which clog the substrate. The owner does not understand why his tree is going down hill while it in the correct substrate. Well, it is not anymore, it has clogged the substrate. Therefore I demand that the original soil from the habitat must be removed as soon as possible. When this is done one can leave the tree in the pot for a very long time. At leas this is what I am doing with very good results.