Background story:

Current Earth, in the close future. Astronomers have found first signs of extraterrestrial life: huge sheet-like objects (with surfaces easily as large as the surface of the Earth) appear all around the sun. They do not approach Earth, but remain in stationary spots relative to each other. And it is nearly daily that new objects appear all of them at the same distance from the sun at half of Mercury's orbit.

After several weeks of observation, it is clear that the sheets cannot be natural events – starting with how they keep their exact position relative to each other and relative to the sun, to them being of some strange mix of trans-iron metals, and ending with sheets not being a natural rock/... formation. They MUST have been formed by aliens; even if we cannot make out exactly how they got transported to the sun. They definitely didn't get towed by spaceships or anything normal like that "“ one day a spot is empty, and the next there is one of those sheets. No clue how they appeared, no radiation anomalies, nothing.

We try communicating with what clearly has to be an alien civilization, but there is no answer to any of our attempts.

Gradually, more and more sheets appear in what is starting to look like a Dyson grid "“ if not the beginnings of a Dyson sphere.

And then, one day, a text message is displayed on all screens on the entire world: "Hello. We are the GGs, the Good Guys, an advanced civilization from the X system. Since you appear to have advanced to technological toddlerhood and seem to be prone to panicking, it has been decided it would be best to inform you of the current happenings. We are the GGs, after all, and if you kill yourselves due to your poor instinct control and backward fear response, we will have failed our mandate of Protecting All Lesser Races.

"Your star system has come to the attention of the BBs, the Bad Boys, who delight in eradicating entire civilizations. They will be in your neighborhood within the next decade, in two Earth years at the earliest. To prevent your planet from befalling harm, we are currently building a sun-powered defensive shield for your entire solar system. Whenever we turn on this shield, though, it will take the entire energy output of your sun "“ meaning that to a Lesser Race like you it will look like the sun got turned off.

"Don't worry, we know exactly how long we can keep the shield going without affecting your planet unduly at your current technological level. We will not be the cause of human deaths, and we implore you to keep a well enough order on your planet to not cause deaths amongst yourselves.

"Toodles,

"The Good Guys"

After a short demonstration of the full functionality of the sun-off switch, humanity actually believes the GGs. They start to wonder about how long they could expect the sun to be turned off at the longest...

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TL;DR

Quick Facts (those who do not want to read the background story):

• Earth: current technological level
• Aliens can and will turn off the sun temporarily. This may be a repeated event. Aliens say they won't turn off the sun long enough to damage the planet or humanity, but don't give any exact numbers.
• Earth has forewarning: earliest sun-out is in two years

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Question

How long can the sun be turned off without planet Earth taking enough damage to endanger human survival? (Assume that the sun will be turned on again after this period of time, and humanity/Earth should be able to bounce back within a few months. The next sun-out will come only once humanity/Earth has recovered.)

There is enough forewarning to establish sufficient food"¯stores and shelters etc., so at least a part of humanity should be able to weather the event in pretty comfortable conditions. Afterwards, sunlight will be just as available as it was before again. However, what about

• Non-human life? How long can plants survive without sunlight? How long until the ecosphere will be damaged so badly that it cannot be... reforested again when the sun turns on again? What about animal life? Species dying out is acceptable, but not to the point of an extinction-level event (less than 10% of all land + maritime species)
• Oxygen? Does this even become a factor due to the restrictions on killing plant-life and temperature?
• Temperature? How long until the average temperatures have dropped more than, say, 10 degrees Celsius?
• Natural catastrophes? Will the sudden absence of sunlight cause huge hurricanes / ice storms /...? What about a continued absence of sunlight?

A ball-park figure would be enough for me; is it a couple of days that Earth can weather only, a couple of weeks, or might Earth even be capable of enduring 2-3 months without sunlight?

EDIT: I am looking for the longest possible time the sun can be turned off in one go. 4 hours daily is not what I am looking for :)