Hello fun4stuff,
This is an area where you will find as many different approaches as there are meadmakers, so rest assured that anything I say subsequently is just my opinion (for whatever it's worth).
Bulk aging is, of course, the aging that goes on before the mead is bottled. How long you bulk age may be based on many factors including the style of the mead, and how much bulk storage capacity you have, whether there are off flavors to age out, and other timing issues that play in. I have let some of my meads bulk age longer just because I was too busy to spend the time to bottle them. There is some debate about whether bulk aging is better than bottle aging - it is certainly more consistent because after it is bottled, each individual bottle goes through changes for better or worse depending on how and where it is stored, etc.
My rule it to follow the schedule of the yeast rather than to follow mine. When I have a batch that I want to backsweeten, I will keep it in the secondary (or tertiary) until it clears quite a bit - not necessarily crystal clear, but enough so that the yeast flavors have cleared and I can get a good idea of how it is going to taste. This can take a couple of weeks or a couple of months. Then I rack it, stabilize and backsweeten.
At that point, I am not in a hurry to bottle it. I want to make sure that the yeast do not start up again. I do not want any bottle bombs to clean up. Sometimes it will clear in a few weeks, but I have had some batches that have been slowly clearing for months. This is where my patience gets truly tested. If you do bottle too soon, you will get nice sediment in your bottles - not harmful, but not pretty. Fining agents and filtration are always options, but usually time and cold storage will have it clear.
When it is clear, then I will bottle it. I have not been doing extended bulk aging beyond the point where it has really cleared, but it could certainly be done. Mostly this is because I want to free up the containers for new batches. I do have a leap year mead that will be stored and aged for four years, and I probably will keep it bulk aging for at least half of that time, maybe more. These decisions are where mead is definitely more art than science.
I'm not sure if this really answers your question, but I hope it helps some.
Good Meading!
Medsen