If your service has existed for T years and you store N objects and have never lost any objects, then at most you can claim "less than 100/(T*N)%" yearly object

If your service has existed for T years and you store N objects and have never lost any objects, then at most you can claim "less than 100/(T*N)%" yearly object loss rate. If you claim any less than that, you have no empirical evidence to back it up. (The more accurate formula would take into account the length of time each object has been stored, since not all objects were stored for the full time period.)