Question
When is the right time to repot dendrobiums?
When in its growth cycle and how crowded should I wait frothe pot to be?

Answer
Dendrobiums like to be pot bound (ie as tight in the pot as possible).  If you wish to pot you should do so when new roots are just beginning to emerge (ie less than 1 inch) to avoid root damage during repotting.  Root damage while repotting might set the plant back and prevent the full development of new growth during its new growth period.