Dendrobium rest period - when to start?

[Ed Weber]

Hello.
My specialty is pleurothallids but I'm becoming enamored with species dendrobiums. I have a Den. jenkinsii (assured not to be aggregatum), as well as a lichenastrum, 2 kingianums, and a mortii, or what used to be called mortii anyway. I have noticed that they start new leads then tend to abort them. I've had them over a year now and know that they need a rest period, I just don't know when that period should start, how long it lasts, and what, if anything, I should do while the plant is resting.
I live in southeast Pennsylvania, zone 5, altitude 846 ft.
Any advise would be greatly appreciated.
Ed