Undecided questions
How long does it take to speak like a native speaker,

Depends on the person, but my friend who had no spanish exposure, moved to south america for work and after a year, he was able to have a basic conversation. After 1.5 years he was decent at talking and 2 years later, he was pretty good. And he didn't really practice much, other than taking spanish classes 2 times a week and using the spanish he learned to go to the market and do some basic errands. Thus, I'm going to guess that if you put in more time and more effort, it might take you less time to be fluent.

I think it is to difficult to speak like a native speaker. But I think it is something that can be done if you do these two things. First, study really hard the grammar, and literature at the highest possible level and then go to the country they speak this language and stay there. let's say for 3 years practicing every day the language and keep reading on the same time. Then, I think it is possible. But this two are to be done both.

I have a neighbor from Australia as you. He is here for 20 years approximately and speaks spanish fluently but not as a native speaker. I understand him and we can have a conversation, but his accent is different.

I think it´s difficult to speak as native speaker in short time, but if you get speak fluent, you won´t have problems.

All your life time.