Article 39 CISG
(1) The buyer loses the right to rely on a lack of conformity of the goods if he does not give notice to the seller specifying the nature of the lack of conformity within a reasonable time after he has discovered it or ought to have discovered it.
(2) In any event, the buyer loses the right to rely on a lack of conformity of the goods if he does not give the seller notice thereof at the latest within a period of two years from the date on which the goods were actually handed over to the buyer, unless this time-limit is inconsistent with a contractual period of guarantee.