Capacity -  An Unlimited Number
Duration -  Life Long

Long Term Memory (LTM) occurs by some type of structural change that takes place in the presynaptic connections, or perhaps in the entire neuron, which permanently facilitates the transmission of impulses at those synapses.

      When all of the synapses are facilitated in a Cell Assembly or Thought-Circuit , this circuit can be re-excited by any one of many diverse signals at a later date, thereby causing memory of the event.   This overall facilitated Thought-Circuit is called a Memory Engram and is believed to involve the Hippocampus, Thalamus, Frontal lobes, and Temporal lobes (for abstract memory). 

      Retention fails when brain functions are disrupted too soon after acquiring new information. LTM requires a mimimum of 5 to 10 minutes and up to one hour (with certain types of information), for retention or Consolidation of new information to occur! 
   This is because the process of consolidation involves coding the new information into different classes.   During this period, similar information that has already been stored in LTM zones, is recalled via the Thalamus for comparison with the new information.
        The new and old are compared for similarities, and for differences, and part of the coding process includes these similarities and differences.   New memories, during consolidation, are stored in direct association with other memories.  The Association Cortex is very actively involved in establishing LTM !