This might work:

// untested
public class IntervalEvalulator : ITriggeringEventEvaluator
{
    private DateTime lastTriggeringEvent = DateTime.MinValue;
    private int intervalSeconds = 60;

    public bool IsTriggeringEvent(LoggingEvent loggingEvent)
    {
        if (lastTriggeringEvent == DateTime.MinValue)
        {
            lastTriggeringEvent = loggingEvent.TimeStamp;
            return false;
        }
        else
        {
            TimeSpan diff = loggingEvent.TimeStamp - lastTriggeringEvent;
            lastTriggeringEvent = loggingEvent.TimeStamp;
            return (diff.TotalSeconds > intervalSeconds);
        }
    }

    public int IntervalSeconds
    {
        get { return intervalSeconds; }
        set { intervalSeconds = value; }
    }
}

You could also create an appender that flushes at a predefined interval:

// untested
public class FlushIntervalSmtpAppender : SmtpAppender
{
    private Timer timer;
    private int intervalSeconds;

    public override void ActivateOptions()
    {
        if (IntervalSeconds > 0)
        {
            timer = new Timer(IntervalSeconds);
            timer.Elapsed += delegate { if (BufferSize > 0) Flush(true); };
            LogManager.GetRepository().ShutdownEvent += delegate { timer.Stop(); };
            timer.Start();
        }

        base.ActivateOptions();
    }

    public double IntervalSeconds
    {
        get { return intervalSeconds; }
        set { intervalSeconds = value; }
    }
}