In the modern context, we can observe that Norwegian and Swedish are mutually intelligible, but Danish is not. Lacking the technology to do the experiment with historical speakers, the only way to figure this out is to see what writers of the time thought. A lot of the records are oral, and written down later, so there is a fair amount of plus-and-minus to such date calculations. It is said that in the Heimskringla, ca. 1230, there is evidence of significant dialect divergence: stirt var honum norrœnt mál, ok kylfdi mᴊǫk til orðanna, ok hǫfðu margir menn þat mᴊǫk at spotti ("the Norse language was hard for him, and he often fumbled for words, which amused people greatly"). The Wiki opines that "From the late 13th century, Old Icelandic and Old Norwegian started to diverge more"; it also reports the Grágás (apparently written down around 1260) as saying that "Swedes, Norwegians, Icelanders and Danes spoke the same language". Finally, "In the body of text that has come down to us from until c. 1300, Old West Norse had little dialect variation, and Old Icelandic does not diverge much more than the Old Norwegian dialects do from each other".
(This video demonstrates perils of trying to make guesses about "mutual intelligibility" (a Danish and a Swedish speaker try to pronounce phrases in each other's languages, and the Danish speaker does much better than the Swedish speaker at getting the other language -- is that because of the languages, or the individuals?)