How long should a take-home exercise be?
Under three hours, with a stated cap and an explicit instruction to stop. Anything longer selects for people with free evenings rather than for skill, which is a filter most teams would reject if it were stated out loud.
What is actually worth asking at the end?
Concrete process questions: how the last incident went, how long the last change took from merge to production, what happened to the previous person in this role. Culture questions get culture answers; process questions get facts.
How do I answer the expected salary question?
With a range anchored on evidence, and the question turned around first: asking what the band for the role is costs nothing and is answered honestly more often than people expect. If the answer is refused, that is information too.