Msgs - Log entries emoved before 30 minutes

I am monitoring /var/log/messages for certain entries. It seems to work fine, except that entries on the msgs page are removed before 30 minutes. They seem to only hang around for about 15 -20 minutes. If you click on full log, you only see entries that are less than about 20 minutes. I thought they should be there for about 30 minutes. Is this configurable? I don't see anywhere to change it. I would like for them to be visible for at least 30 minutes, longer if possible.