Lucene NRT: When to commit?

We're refactoring our Lucene host (Lucene.NET 2.9.2), and are implementing Lucene NRT (Near Realtime).

What is th开发者_如何学Ce best time/threshold to commit the changes to disk? Is there a golden rule? If it is when the internal ramdisk holds a certain amount of data, how do I get the size?

Once a commit happens we update our database, so I'm not that fearfull of power failures (once the process starts again, it will reindex those documents that have not been committed).

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I have just implemented what sounds like the same scheme in our system. I decided to do a commit when I have over 1000 uncommitted documents. I think the number really depends on how many docs/sec you will be adding. I am also not sure if I can run the commit on a different thread than where I am adding the docs.