Mutual intelligibility is difficult to measure since speakers of two different tongues could meet each other and hardly understand each other at first but after a week of close contact, they can understand each other quite well.As far as intelligibility goes, it is usually measured blind with only one group at a time. It is uncertain where to split dialect and language, but Ethnologue (SIL) seems to generally split at 90%. Above 90% = dialect. Below 90% = dialect.
With two separate but closely related languages such as Turkish and Azeri, after 3-4 weeks of close contact, they can communicate quite nicely. I would put 3-4 weeks at the barrier of dialect and language.
At the other end, in Africa, speakers of various lects talk of one day languages and two day languages, referring to how long it takes speakers of Lect A to understand speakers of Lect B. These 1 day languages and 2 day languages are best seen as dialects of a single tongue.
Closer to home. it takes one day of close contact for other Spanish speakers who land in San Salvador by plane to completely understand Salvadoran Spanish. It takes Argentines three days to understand Chilean Spanish. So we can call Salvadoran Spanish and Chilean Spanish dialects of the Spanish language. Salvadoran Spanish could be called a 1 day language and Chilean Spanish could be called a 3 day language.However, with Canarian Spanish and Dominican Spanish of the Dominican Republic, it takes other Spanish speakers about three weeks to catch onto it. So Canarian Spanish and Dominican Spanish are like Azeri and Turkish. I honestly think that Canarian Spanish and Dominican Spanish are separate languages on MI grounds, but it would cause a political firestorm if you tried to split them so no one will.
In Spain, there are various lects such as Asturian, Galician and Andalucian. A Spanish speaker may take two months or so of close contact to learn to understand Asturian and Galician well, and indeed, both are listed as separate languages.Some Spanish speakers report that Andalucian sounds absolutely insane when they first listen to it and they can hardly understand one word, however, after 2-3 hours of steady close listening, they can understand it quite well. We may call Andalucian a 3 hour language and clearly Andalucian is a dialect of Spanish called Andalucian Spanish.
Once it starts to take as long as 3-4 weeks of close contact for speakers of Lect A to understand Lect B, I think we are looking at two separate languages. Anything less than that, starts to seem a lot more iffy.
