Each witness generally will use different sources and settings and intervals to update their price feed. They can set any price they want, although now there will be more scrutiny on accurate reporting. Any automation of price feeds is done outside of the steemd software, although the command is fed to it. The command looks like this when using cli_wallet, if you are curious: publish_feed "pfunk" { "base":"0.225 SBD", "quote":"1.000 STEEM"} true