[SURBL-Discuss] multi.surbl.org.rbldnsd Update Times

I'm sure this is dependent on the mirror we are pulling the zone from, but how often is the source zone updated, and should we expect that mirrors update on a regular schedule around the clock? What I'm getting at is we are monitoring file changes to the zone file to ensure it's updating often, but as of Friday we have received no updates (I'm guessing because it doesn't get updated on the weekends). Is my assumption correct? Does the schedule vary by mirror? If so is there a mirror that anyone knows the replication interval for? If not Jeff do you know the schedule requirements for a site to become a mirror so we can base monitoring off this schedule?

Any input is greatly appreciated. Thanks, Eric