Implement a data structure which allows key-value elements to be added and retrieved. The data structure should fulfill the following requirements:

• It is possible to add and retrieve an element;
• Adding an element replaces a value whose key already exists in the structure;
• Elements are added with a Time-To-Leave expressed in millliseconds;
• Once the Time-To-Leave of an element is reached, it is considered to be non-existent;
• The structure has a limited capacity defined in the constructor call;
• An element whose Time-To-Leave is lower then the lowest Time-To-Leave in the structure should not be added.
• When adding an element to a full structure, the pair with the lowest Time-To-Leave shall be replaced if there's no other entry with the same key already stored;
• The data structure provides information about its current size.