Is there a rule when to commit changes to the data base?

For examples i want to save 20 objects. Could i do that w/o doing any commits in between?
What is a good rule of thumb?

Thx for any hints
Klaus

Some basic information to that topic:
http://www.tutorialspoint.com/hibernate/hibernate_batch_processing.htm

There is no specific rule. Usually you have a requirement that several changes need to be part of the same commit.

In some other cases, you are forced to make smaller commits because including too many changes in one large commit might make it too slow.
Generally speaking, each commit has a performance cost so if you have to write just 20 objects and these are not huge, you should include them all in the same commit.