When bulk aging, do you use a airlock or air tight plug?
Depends on a number of factors. Has the mead been stabilized or if there a chance that fermentation could begin again? Is it in a location with a steady atmospheric pressure, as a change in atmospheric pressure will guess gas in the mead to go into or out of suspension which could cause the cap to blow off or the carboy to shatter? Air locks seem to be the safer of the two options, just remember to keep the water level in it topped off, but others on here have used solid bungs (and have also had those solid bungs blow off leaving the mead exposed to oxygen).
Bulk aging between six months and 2 years.
Depends on why you are aging it.
Till it falls clear, if you waiting for it to clear.
Till the flavors meld together and alcohol heat receeds, if the taste could use some work.
Taste it when you think it is done fermenting and start to bulk age it. Take notes as to what you want to happen. Look at it and/or taste it every once in a while.
To naturally degas & clarify without clay.
I usually add bentonite & degas 2 days after stabilizing & let it sit for another 3 weeks before bottling. I wanted to see which is better. If bentonite & degassing is better, I will stick with that.