I believe the OCSP server updates almost immediately when a certificate is revoked. New Good repsonses are generated approximately daily, but the nextUpdate for them is always 7 days away, so these responses can still be used for purposes like OCSP Stapling for that 7 days even if the server generates a new response in the meantime.
You probably saw an update happen earlier than you expected because there was a massive outage the night before last. The operations team may have forced the regeneration of all OCSP responses as part of getting things working, or they may have shifted traffic to a different datacenter that produces different responses.
The reason there is not a fixed schedule is precisely because emergencies like these would render it invalid, and because a revocation causes an immediate update anyway. I'm not really sure how a fixed OCSP update schedule would be useful, anyway. Care to explain?