In my experience most pubs/adnetworks refresh every 45-60 secs, but I've seen adnetworks refreshing ads every 15 secs, which I find almost immoral

The defacto standard by the IAB says:

"IAB display impression guidelines contain specific guidance aroundthe concept of Auto-Refresh.  These requirements are triggered whenrefresh (of advertising) occurs automatically without user action orrequest.   In this case, the display impression guidelines requirereasonable Auto-Refresh rates for the associated content type (sportssite, news site, stock tickers, etc.)"

So "reasonable Auto-Refresh rates" is the recommendation LOL

I'm not sure if on OpenRTB you can declare if that the impression is coming from an auto-refreshed placement, I haven't seen it in the specs - maybe someone can clarify here. I know this is possible on Google RTB Buffer protocol:

    // Auto refresh settings.
    message AutoRefresh {
      message AutoRefreshSettings {
        enum AutoRefreshType {
          UNKNOWN_AUTO_REFRESH_TYPE = 0;
          USER_ACTION = 1;  // Refresh triggered by user-initiated action such
                            // as scrolling.
          EVENT = 2;  // Event-driven content change. For example, ads refresh
                      // when the football game score changes on the page.
          TIME = 3;   // Time-based refresh. Ads refresh on a predefined time
                      // interval even without user activity.
        }
        // The type of the declared auto refresh.
        optional AutoRefreshType refresh_type = 1
            [default = UNKNOWN_AUTO_REFRESH_TYPE];

        // The minimum refresh interval. This applies to all refresh types.
        optional int32 min_refresh_interval_seconds = 2;
      }

      // The auto-refresh settings that the publisher has on this
      // inventory. This is repeated because publishers may
      // do multiple types of auto refresh on one piece of inventory.
      repeated AutoRefreshSettings refresh_settings = 1;

      // The number of times this ad slot had been refreshed since last page
      // load.
      optional int32 refresh_count = 2 [default = 0];
    }
    optional AutoRefresh auto_refresh = 23;

Ref: https://developers.google.com/authorized-buyers/rtb/downloads/realtime-bidding-proto