Have you checked the PADI (or PADI pro) website mate? They are usually pretty good about letting you know standards and such.
But back to your question, what do you mean by retrain. Are you wanting to know how long you can be a lapsed DM without needing to retrain as a DM (ie. work as a DM). I guess you know you'll have to be paid up (in Active status or whatever they call it).
With instructors I don't know the exacts but you have certain bands eg. 1 year out of training: need a brief tune up and check of skills, 2 years: Need some more in depth work with a Course Director, 5 years Need to redo the IDC, more than 5 need to redo IDC and IE.
The above are probably completely wrong but that's the sort of way I understand it to work. Don't quote me though and do check the PADI standards first
Quite probably there is something similar for DM's though perhaps not as in depth as there is less teaching involved in the DM qualification.
I would have a glance at the PADI site for you mate but my internet is playing up