Timer triggerBatchTimer = new Timer(() => yourBatchBlock.TriggerBatch());

TransformBlock timeoutTransformBlock = new TransformBlock((value) =>
{
    triggerBatchTimer.Change(5000, Timeout.Infinite);

    return value; 
});

timeoutTransformBlock.LinkTo(yourBatchBlock);

yourBufferBlock.LinkTo(timeoutTransformBlock);