No one here can tell you when any particular Mac model will appear as a refurb, as there is no published schedule for that. And, it relies on returns/repairs, and how quickly they are made available for re-sale.
You just need to keep checking...
If you're referring to the latest model released, they've already appeared in the refurb store. When a new model is released, they can show up as refurbs anywhere from several weeks to a month or more later. If you're looking for a particular model, you can use refurb.me to be notified when it appears.