When I add a new item in my Libraries or Lists, when I search them they won't appear in search results. And after 3 or 4 days when I search them they appear in my search result.
Why this delay happens?
Item appears in search only after SharePoint performs a search crawl, so when you add a new item, incremental crawl is not yet performed, so the item is not shown in your search result
The time it takes before changes appear in Search depend on the Crawl Schedule for the content source.
How much time it takes depends on amount of content, type of content, content location, processing power, load. You can see how long the last crawl took on the Content Sources page in central admin.