Complex sentence: The Adelaide Festival of Ideas has been held every two years since 1999 over a period of three or four days in October (as of 2011, previously July) at venues throughout Adelaide in the alternate year to the long-established Adelaide Festival of Arts.
Split into simpler sentences: The Adelaide Festival of Ideas has been held every two years since 1999, over a period of three or four days. It is held at venues throughout Adelaide in the alternate year to the long-established Adelaide Festival of Arts.