Generally, I count aging when it's clear and after you rack it to another container and end up with no sediment after a week or so. Then is when I count for bulk aging. Usually, I am shy on carboy space for new batches so I end up waiting a month with no sediment and then bottling. Then let age in the bottle for 6-8 months, some for a full year. I typically wont open up a single bottle unless it has had at least 8 months aging time. At that time, I consider the mead still "Green" and barely drinkable.
If your bulk aging, I would simply rack over and then check it in a month. If no sediment or a very little dusting, then consider that the start of aging. There is no perfect recipie for aging, each Mead needs it's own time. Be patient. Or in my case since I bottle age, Lazy. You see, I bottle it and date it and mark what it is, then forget about it as I move on to new projects. Keeping a Brew Log is also what I do, so I can check on it later.
Oh, don't get me wrong I do anticipate good brew but it is important to just let it age. Mead gets better with age. I have never had a Mead take less than 6 months to age. That one was a heavy citrus one and even that benifited by longer aging.
Matrix (Complex yet simple)