We have no idea how far we are from simulating a brain. Because we have no idea how exactly computation is performed in a brain. Neither in a locust's brain, nor in a human brain.

On the other hand, we have already built systems (ANN based) which can do non-trivial things: play Atari games, tell a cat from a dog, convert speech to text, translate from one language to another, etc.

This points to the strong possibility that all that biological complexity in neurons is completely irrelevant to the principles of intelligence, just like the fact that a modern transistor needs 500 parameters and a ton of complicated equations to describe its physical operation is irrelevant to its main function - a simple ON/OFF switch. If we want to simulate a computer, using more complicated transistor models gains us nothing.