Dean "Iksar" Ayala has announced that these resets are planned to be recurrent, a bit like the Standard mode in Hearthstone.
The first rating reset would take place in three months, and a new one could take place every four months thereafter. Since this is the first time such a change is planned for the Battlegrounds mode, Ayala mentioned that other modifications could happen in the meantime.
Alaya gave several pieces of information on Reddit: the reset concerns the general ranking, and not the MMR (MatchMaking Rating) of your account.
To clarify: it's the visible rating of your account that will be affected. Your MMR, which determines the opponents you face and which actually defines your level, won't be affected. If your MMR is very high, you will level up very quickly. It's the same system as in Standard, where you earn a certain number of stars each month depending on your final level. A Legend player (thus with a large MMR) will level up infinitely faster than a player who has finished Gold or Platinum.
Original content by Jean-Vincent "Durvil" Cannata.