If you have a 100Ah battery at the C20 rate and only use 20Ah per day you could say you have 5 days with no charge until 100% discharged at 10.5V. This is also referred to as 5 days of autonomy. You have actually discharged this battery slowly over 5 days which is 120 hours, so the actual power available might of been 120Ah at the C120 rate. So in theory your battery might still have a little bit of energy left in it until it reaches 10.5V.
Not to say you should keep discharging to 10.5V as that's a very deep cycle to 100% Depth of Discharge.