The simple answer: When fresh rooting activity is expected (generally in the spring) or is very evident, generally every one or two years. Fresh rooting activity is best shown by the nice green root tips on plump white roots. Often, the main "flush" of rooting will come from the base of the plant (in the case of phalaenopsis), or from the developing newest growth (in the case of dendrobiums and other orchids with pseudobulbs.) Orchid plants need repotting for one or a combination of two main factors: Potting mix breakdown, often evidenced by dead roots; or plant over-growing the pot, growing over the edge. In the first case, a larger pot may not be required, simply replacement of the growing media. In the second case, the plant may require dividing or may simply be shifted into a larger pot. Fresh media should always be used. A good general rule of thumb is to pot for the bottom of the plant, the root system, and not for the top, the foliage. Freshly repotted plants should be placed in a shady, humid area until continued new root growth is observed. In general, if in doubt, pot in the spring.