What is a reasonable length of time to ask wind/brass players to sustain a note?

20 seconds? 30? 40?

Completely dependent on range, dynamics, and instrument.

if it's a solo piece, no more than 20 seconds, and that's a lil much I would say. We can do it up to about 25 seconds but just because we CAN do doesn't mean write it! I normally say no more than fifteen for high-end stuff and ten seconds for easier pieces.

if it's ensemble, we can stagger so you wouldn't notice!

Well, oboe players can sometimes hold it for more than a minute. If it's a solo piece, don't do more than 15 or so. In large orchestra, they can breathe without others noticing.

Completely dependent on range, dynamics, and instrument.

Exactly,

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First, never forget all people must breath :D even musicians, may sound obvious but I'll tell you, sometimes you still have air in your lungs but you have to release it because you NEED to breath again...

I will only answer about the winds I play, (which are only 3 :P )

Clarinet:

ppp (as long as you can hold by taking a deep breath)

pp 30 seconds

p 25 seconds

f 20 seconds

ff 18

fff 15

Tenor Saxophone:

ppp (there is no ppp on tenor unless...) ¬

ppp (breathy tone) 8 seconds (less lip pressure needs more air)

pp (breathy tone) 10 seconds

p (normal) 18

f ( " ) 15

ff ( " ) 10

fff ( " ) 8

Soprano Saxophone:

ppp 25

pp 20

p 20

f 15

ff 15

fff 12

Of Course, don't use these numbers as overall rules, you have a an idea.

ah thanks everyone, I'm writing for chamber orchestra, it has 1 of each wind and brass

Take a deep breath. Now blow. There is your answer. But of course, you have to take inconsideration how much breath you an instrument needs to sound, and how much it takes for it to continue to sound. So a brass instrument is probably hard to keep sustatined than, say, a clarinet!

20 seconds? 30? 40?

There's no real general rule. All of these elements factor heavily into it.

Is the instrument doubled?

How loud is it?

What range of the instrument is it in?

What instrument is it?

How does the note lay on the horn? (Some are hot notes that don't come out as easily)

Are catch-breaths out of the question?

Is the player professional, amatuer, or hs/ms?

Your best bet is just to learn by trial and error.