Art. 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.
The Secretariat Commentary on Art. 37 of the 1978 New York Draft, predecessor to Art. 39 CISG, gives insight into the drafters' unterstanding on the eve of the 1980 Vienna Diplomatic Conference.
CISG-AC Opinion regarding Art. 39 CISG
Overview of case law on Art. 39 CISG: UNCITRAL Digest
The UNCITRAL Digest on the CISG can be used as a starting point to discover the relevant case law on this article.