SpaceX launched two GEO comsats, and is about to launch two more, that use all-electric ion propulsion.
Now, I'm wondering, how much time would it take to actually make a satellite like that operational. After all, while ion propulsion is cool and lightweight, it has a very, very low thrust. (Eutelsat 117 West B apparently has only about 200 mN — that's millinewtons — of thrust)
Obviously I can just look it up for an operational satellite, but would be interested in calculating the answer for any arbitrary satellite.