In order to plan the Incremental and Full Crawl scenarios and schedule , I’m adding the below information in this regard:                                                                 

• Incremental crawl covers the following scenarios, where SharePoint index files will be updated and search results will affected without the need to perform full crawl:
  • Adding new document to document library or new item to list
  • Edit current document content
  • Delete document (incremental crawl will detect the deletion and removes it from the index and from search results).
  • Changing Item permissions.
  • Document properties (columns)

Note1: a full crawl should be done at least once before performing incremental crawl.

Note2: the above scenarios are covered for crawled content within SharePoint 2010/2013 farm, (not file shares , exchange public folders, custom connectors , etc… for those scenarios incremental crawl behavior might differ).

• Full Crawl includes the following scenarios :
  • If the incremental crawl failed to crawl content for errors , Incremental crawl removes the content from index  until next full crawl.
  • When a software update for SharePoint or service pack is installed on servers in the farm, full crawl is required.
  • Add a new mapping for the Managed Metadata property which used as refinements in search results. (e.g. add new filter in search center for documents to refine results by division ).
  • To include document changes on a non SharePoint 2013 sites or older versions of SharePoint, like ASPX pages on Windows SharePoint Services 3.0 or Microsoft Office SharePoint Server 2007 sites.
  • You want to detect security changes that were made on a file share after the last full crawl of the file share.
  • Crawl rules have been added, deleted, or modified which is used to include or exclude sites URLs to be crawled.
  • Creating  server name mappings.