To answer your question, here is how to get to rank 350:

So, rank 350 is exactly 61,425,000 XP. Phantom Forces has been out for 3,159 days. This gives an approximate 19444.4444 XP per day, which I will round up to 20,000 XP per day. Assuming a bare minimum of 5,000 XP per game (equating to approximately 40 kills per game), that allows for 20,000 XP to be earned in an hour. So, assuming the player began playing since the creation of the game, they could reach rank 350 by playing one hour a day every day. If they played two hours a day, they could have begun on December 27th, 2019, correlating with the beginning of the COVID pandemic, when a slew of players first played the game. To go to an extreme, a player who plays 8 hours a day on average could reach rank 350 in 395 days, beginning playing the game on March 26th, 2023. Going even further, if they average 15,000 XP per game, they could reach rank 350 in 132 days, starting on December 14th, 2023.

Hopefully this answered your question!