Originally Posted by ExitRowAisle

I'm not sure I completely understand what you are asking, but here it goes:

At ING, on $10,000, you should earn daily interest of $0.82 (10,000 x 3% / 365 days). If you don't withdraw the money, this daily interest should increase with compounding. If you withdraw it partially or completely, you only earn daily interest on your remaining deposited balance.

The comments above seem pretty straightforward -- so perhaps I have misunderstood your question about "gaining interest . . . based on longevity."

I've never invested in a bank that offers miles, but if their computations are based on average balances, you can't game the system by timing large deposits. Averaging will negate the timing effect. If the bank's computations are based on measurements on specific days (e.g., end of month), then theoretically, you could game the system by timing deposits and withdrawals. Someone else might be able to shed some further light on this subject.