How do I go about setting a drive to spin-down when not in use?
Depends on OS. I recommend you search these forums as we have discussed this many times.
I remember that for Windows, revoSleep is pretty well thought of, but I think I remember reading somewhat recently of an even better program.
How quick does it spin backup again when needed though?
< 1s. Long enough that it is noticeable if it is something that would open instantly from a spinning hard drive, but I doubt you'd notice the extra time for a big file that takes a few seconds to open anyway.
It is important to note that once your drive wakes up you should probably have it stay awake for some time before putting it back to sleep. Spinning up is the most stressful activity for a drive. How long depends on the drive. Things like a 2.5" drive (non-Velociraptor) are meant to be put to sleep quite often to conserve laptop power, so I'd say as little as 5 minutes. WD Green are also meant to be powered down quite often -- maybe 10-15 minutes. For normal desktop drives, I'd think 30 minutes or longer would be best.