I think Matthew 24:14 tells when the end will come. However no one except God will know when the Gospel has been preached to the whole world. I guess when the last person has heard the Gospel then the end will come.
Some scholars have said that in the original, GREEK, it indicates that ALL PEOPLE GROUPS, tribes, must be exposed to The Gospel.
About 18 years ago, I heard a prediction that considering the newer pattern of translating Scriptures orally and distributing them on cassette tapes; as well as the pattern of many languages dying in favor of more broad based languages like English--it would take another approximately 30 years to get The Gospel to all remaining language/people groups.
Perhaps we have 12 or less years left on that score. The process was being speeded up and I haven't heard a current projection. It's probably available on the web somewhere.