Is anyone aware of a Military Regulation that states you must receive your orders so many days in advance before leaving for a reservist?

There is no regulation that I am aware of. Being former reserve it was always my understanding that they were to be given "as much time as possible, so as to inform their employer" but nothing in so far as a defined time frame.

Usually we had ours a month in advance but one one I recall we had two days notice. Luckily command was always good at putting out an advance outlook that soldiers could take to their employers as to plan, prior to orders.

I’ve received orders as early as over a month before a school or as short as a week before. I’ve had other soldiers receive theirs the day before.

Usually you know though when you are expecting orders and can let your workplace know. I am currently awaiting amended orders for next month. I know what the dates will be so I let my work know and will get them orders ASAP. It’s just part of the gig with being a reservist. Hooah Gumby.

I doubt there is any regulation that gives a specific number of days prior. More likely, you will find general policy that states something along the lines of, "Soldiers should be given as much notice as possible when it is necessary for them to be placed on military status, in order to ease the burden on their personal lives, civilian education, and employers..." Blah, blah, blah.