I'm in the same boat as you, Brother... 90 days or, possibly (thought doubtfully) less, from the date the letter of notice was executed:
"Please note that you have 90 days from the date of this letter or until the Board issues a decision in your appeal (whichever comes first) to request a change in representation or to submit additional argument or evidence, if you elect to do so. Any such request or submission must be sent directly to the Board. See generally 38 C.F.R. § 20.1304."