Charles -
There is no testing schedule as far as I know. I've been in this area for a little over 3 years and it appears the siren has been here since at least 2004. The first time I was aware of it (and what attracted me to these boards) was when it was used in April.
Here's an article talking about the sirens from 2004:
http://www.thefreelibrary.com/JAIL+LOOK ... 0113476047
I'll have to take one of my off days and go locate and log locations of the sirens in Castaic. I'm a full time photographer, so you can bet there will be some very nice photos.
Also - I believe the siren here is also/is referred to as an ASC PN-20. Not sure which name is correct.