Need explanation on virus definition dates and releases, including SONAR and IPS Signatures

I hope somebody can assist by answering or sharing a link that contains the info I require. I tried searching the net but cannot find the explanation I'm looking for.

I need to find out why Symantec only releases virus definitions every second day and the meaning behind the releases.

I understand what SONAR is used for, I need to find out why the updates are so far apart, for instance:

Latest update will be 2012/11/30 r11 and the previous is 2012/10/31 r11 (example)

And the same for IPS Signatures, why isn't it every second day like virus definitions?

Any explanation to the above would be greatly appreciated!

Regards