Question about "shards"

So, if I understand correctly, there are different "shards" in the game that contains all of the players that started around the same time as you, right? This is easily seen in the Squad Arena. Proof of this is that I am rank 40ish in mine and a guild member of mine is around the same rank, but I don't see him in my top 50 along with me. This creates fairness amongst the player base due to how much time you have (to spend money) in the game.
So the question is, when does one shard cycle start and end? Is it month by month? Week by week? Or once a certain amount of new players is reached then it starts a new one? (Like 5,000?) The reason i ask is cuz I have a friend who wants to start and I want to make sure that he starts at the best time to rank up higher as he'll be a strong and dedicated player who is even willing to pay.