How long does it take to steam brown rice?
This is an impossible question because it depends on the rice you are using: Some rice cooks in 10' other rice in 20'. That's a huge difference. 

Also, some people like the rice to be slightly undercooked and others like it very overcooked. 

At any rate, if you are used to boiling one type of rice for some amount of time, try steaming for about 20%-25% longer and see what you get. 

You can try the rice by taking a grain and looking at it. If it has a white dot in the middle it's not done. When the white dot disappears, take a bite and see for yourself whether you like it.