When should I use Apache Solr instead of Drupal core search? I'm in the process of migrating a medium-sized on-line magazine to Drupal. 

I do not know much of Drupal's internal search engine capabilities, but I'm aware of the Apache Solr Search Integration module, which integrates the java-based search engine from Apache. Unfortunately, Solr is something new to me too.

So my questions are:


When should I use Apache Solr instead of the Drupal core search? In what type of websites core functionality is not enough? Should I care about the performance gains of using Solr if my visitors only do hundreds of searches per day?
Will learning how to use and integrate Apache Solr cost me lots of weeks/months?
Learning and integrating solr could take no more than a couple days. There is ton of documentation on the API and a wealth of resources on Drupal.org.

Your use case sounds like a great candidate. Here are three good reasons I would use it on a particular site.


You have a significant amount of searches being made from the site and limited server resources, Solr will increase efficiency.
You need specific search control beyond core, core search really is just search content inside of nodes, It will ignore block, pane, etc. content. (There are other options that dont require non-php based environments).
Search is a major part of your site. It is common that users are finding content through google and other major search engines so search is not as important as it once was. However, some sites encourage or even require the user to search the content of the site. For anything like this, I would suggest solr, in addition to the other listed items, SOLR offers more control and search power (facets and related content) and less limitations.