I dont know for sure but i think its by deposit.
One time i deposited some BTC and my level went up.
Have no other clue.
Level 0: L0 Your Net Balance is 0 BTC //0.0BTC
Level 1: L1 Your Net Balance less then 0.2BTC > 0BTC
Level 2: L2 Your Net Balance less then 0.5BTC > 0.2BTC
Level 3: L3 Your Net Balance less then 1.0BTC > 0.5BTC
Level 4: L4 Your Net Balance less then 2BTC > 1BTC
Level 5: L5 Your Net Balance less then 5BTC > 2BTC
Level 6: L6 Your Net Balance less then 10BTC > 5BTC
... and so on