To my knowledge there is no set schedule. You can pretty much count on new signatures coming out no later than a day after Micro$oft release security updates in response to newly published vulnerabilities. New sigs also come down in response to new viruses, worms, Trojans and other exploits.
If you want to be proactively notified when new signatures come out please go here.
http://www.cisco.com/pcgi-bin/lm/show_form.pl?111144874980187988281257448&123668_4
Hope this helps.
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