1. Put your plastic milk container into your kitchen sink (or even better a half-sized side sink if you have one)
2. Run your water until it gets as hot as it will go
3. Put the plug in and fill the sink up so that the milk container is floating (the more water the better, so fill it up as far as you can without risk of overflwoing)
4. Go and do something for (less than) 30 minutes or so and retun to some defrosted milk
This works well because not only does the hot water quickly convect heat from the frozen milk, but it also normalises the temperature between the two fluids and hence keeps the milk cold. Hence, if you forget about it for a while, you can trust that it will have at least kept cold for longer than if you have left it out on the counter.