You have to know that what you're hearing from the Scientists are just their theories on what will happen. No-one has ever confirmed anything they've said. That's mainly because no-one really knows what will happen. Some say there will be an eclipse, some a planetary alignment, some say the sun will emit waves of radiation that will set the Earth on fire, others say the waves will knock out the power all over the Earth putting us back in the Dark Ages. I also heard somewhere that the sun will get so hot that it will make some people go insane and they'll go around killing and eating people (something like Zombieland, lol). Also heard a planet called Planet X will collide with either the Earth or the sun causing either one to explode. And some say the sun or the Earth will just simply explode.
But to answer your question, there is no date on how long the darkness (if that happens) will last. Probably from a few days to a few years. If I were you, I would just do what my family and I are doing, stock up on guns and ammo cause if there is total darkness, people will go crazy and begin looting and breaking into people's houses. There is already suspension the government is going to head underground and let us just fight it out. Hopefully, it will not come to that, but giving this country, probably will.