It's hard to say how long it would take for Artificial Neural Networks to achieve human capability. Artificial Neural Networks, even though they are loosely inspired by neurons, still function very differently from a human brain. You would also have to consider the number of neurones, the way they are organized, and the way they learn. There are still many unknowns about the human brain, we can't replicate it exactly. Even if we manage to reach human level intelligence with ANNs, they might still work in a completely different way. They could perform bettern in some tasks and worse in others. It also depends on how you define "how long". An AI might need more examples than a human to learn something, but also be able to read an entire book in just a few seconds. 
In conclusion, there are too many unknowns to estimate the time an ANN would have to spend learning before it reaches human capability.
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