We estimate that each day in transit is equivalent to an ad-valorem tariff of 0.6 to 2.1 percent.
That is from David L. Hummels and Georg Schaur, the AER version of their paper is here, various ungated versions are here. If you are wondering, parts and components are the most time-sensitive goods in international trade, according to the authors. Note also that airplanes are increasingly important for international trade:
From 1965–2004, worldwide use of air cargo grew 2.6 times faster than use of ocean cargo.