In some cases you may see crawls never end, such as when you upgrade Sharepoint to a later version. In such cases you want to check the health of the search service application. 

Checking the health of your search server service application

1. Open SharePoint 2010 Central Administration.
2. Click Manage service applications.
   
3. Click Search Service Application.
   
4. Scroll down the page to see the applications components. Identify if they are in an Online status.
   Sharepoint 2010
   
   Sharepoint 2013
   
   
5. If you see any failed components, check the Search Server logs for exceptions.
   %ProgramFiles%\Common Files\Microsoft Shared\Web Server Extensions\14\LOGS