I often see people schedule full crawling weekly that consume lots of resource and affect the performance.
So when do you need conduct a full crawl?
A software update or service pack was installed on servers in the farm. See the instructions for the software update or service pack for more information.
A Microsoft Office SharePoint Server 2007 shared services administrator or SharePoint Server 2010 Search service application administrator added a new managed property. A full crawl is required for the new managed property to take effect immediately. If you do not want the new managed property to take effect immediately, a full crawl is not required.
You want to re-index 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.
You want to resolve consecutive incremental crawl failures. If an incremental crawl fails one hundred consecutive times at any level in a repository, the system removes the affected content from the index.
Crawl rules have been added, deleted, or modified.
You want to repair a corrupted index.
The Search service application administrator has created one or more server name mappings.
The credentials for the user account that is assigned to the default content access account or a crawl rule have changed.
The system does a full crawl even when an incremental crawl is requested under the following circumstances:
- A search administrator stopped the previous crawl.
- A content database was restored, or a farm administrator has detached and reattached a content database.
- A full crawl of the site has never been done from this Search service application.
- The change log does not contain entries for the addresses that are being crawled. Without entries in the change log for the items being crawled, incremental crawls cannot occur.
here is the link to TechNet that details Plan for crawling and federation