It really depends much on your abilities and motivation and spare time that you can spend on learning. There is an American actor Odin Bairon who learned Russian in 5 years so well that I can't even hear accent when he plays at the stage. He is really good. In my opinion 2 years is enough to be fluent if you have strong motivation and self control. But it is better to ask the foreighners how long it took them to learn Russian.