March of this year is asinine. As many posters have noted, there is no beta yet, and that would be five months after announcing it, an unprecedented event. Not a chance.
Given that expansions usually last around two years, how long beta length lasts for, and time of last content patch to expansion release:
I am going to guess beta will begin in February at some point, will last for roughly 5-6 months, and BFA will launch in September. That has it fit around the two year expansion date, the 5-6 month beta, and has it come out 10/11 months after it was announced.
The EARLIEST I would expect it is July. That would be one of the fastest releases they have ever done, a very short beta (unless they begin this month), and a very short final expansion tier. (Antorus was November 28th, a release date of july would mean the last raid was ~7ish months, whereas september puts it at 9 months). However, they seem to have been on point with shorter raid tiers, perhaps the expansion will follow suit. Just that they've promised faster expansions before, and they always seem to be later than sooner.