Ask 10 people and you're likely to get 10 different answers!
With our daughters, we started offering sippy cups at about 6-8-10 months. However, both of them still wanted the bottle just before nap-time or going to bed. With our oldest, we have a fridge in our laundry room that we keep 8 oz. bottles of water in too - we opened it one night and said, "Do yo want the good stuff or water?" She chose water, and that was the end of it.
Our 18 month still wants that last bottle at night though, but drinks from sippy cups the rest of the time. We will still keep offering her sippy cups with milk or water until she decides she's ready, just like our other one did.