Long-term memory is different from working memory or short-term memory, which both last from seconds to hours.
Short-term memory is a temporary potentiation of neural connection that can become long-term memory in the process of memory consolidation.
As long-term memory is subject to fading in the natural forgetting process, several recalls/retrievals of memory may be needed for long-term memories to last for years. Individual retrievals can take place in increasing intervals in accordance with the principle of spaced repetition.
Sleep is considered a necessary factor for establishing well-organized long-term memories.