The rich man and the 1000 casks of wine

$10$ slaves can do it in $5$ hours. The key is that $2^{10} = 1024 > 1000$.

For a solution, see a hint:

Work in binary.

For more details,

Number the casks and label the slaves according to places in binary; the slaves sample a cask according to whether there is a $0$ or $1$ at their position. The status of the slaves after $5$ hours determines a number in binary accordingly, with $0$ and $1$ representing dead and alive, respectively. The cask with the corresponding number is poisoned.