Right, in the spirit of starting as we mean to go on, homework

[NoComparison]

DS1 has his first piece of YR7 homework - he has to write about what an alien visiting earth might feel/experience.

Whilst recognising that quality may well be more important that quantity, I have an 11 yo DS who will write as little as he thinks he can get away with, so how much should he be writing for this type of task?

That is very vague.
The teacher should tell him what is expected. Try to probe further, sometimes the teacher will say spend x amount of time on this work, or they will say two pages or (usually for older pupils) a number of words.
Otherwise there will be some who do two lines and one girl in the class who writes 2000 words .

Hmm, but 20 minutes of him going "I don't know what to write" won't produce much

He got really good results in his KS2 SATS and I have no idea how

He should write what he can, unaided. To the best of his ability. First homework is a very useful piece of information to e teacher.

tiggytape sorry to disagree but I think that is way too much parental "help" for year 7.
I think by secondary school the parental role is to police the doing of the work but not contribute to the content.

English teacher here. I would normally expect a Y7 to spend 30 minutes on a piece of homework and I would expect a child who got really good results in his SATs' to be able to produce at least three or four paragraphs in that timeframe. Don't go with Tiggytape's suggestion, tempting though it is when they are doing the 'I don't know what to write' thing. The homework needs to be their own unaided effort if the teacher is going to assess it properly.

ds managed 35 words for his first homework

The homework is accompanied by some interesting pictures (downloaded from the Internet) and he's clearly spent time thinking about the font. Another

He got OK SATs too (another one here wondering how).

I'm hoping the teacher will tell him to buck his ideas up; he certainly won't listen to me.

I thik thirty minutes is adequate.
But I would insist that DC check their work after their alloted time.

If they really can think of nothing to write, I'd brain storm over tea. But I'd be very suprised if they hadn't already done that in class.

I would imagine the teacher told him how long to spend on his writing . Generally 20-30 minutes is the norm I believe . As a boy imagining he's an alien should be right up his street . I'd suggest he spends 5 minutes thinking about what to write and jotting down a few ideas then just get on with it. Guess the teacher just wants to see how imaginative they can be , what vocabulary they have and how they can express themselves in writing .