Interesting topic. To be truly "perfect" in anything I'd say takes a lifetime. A fair degree of fluency? My brother's neighbors moved here (to the states) from Italy, and their young children (they're around 5 or 6) picked up English in literally (so I'm told) just a few months -- definitely no more than half a year. I have a close friend from China who has lived in the states more than a decade and still speaks with a very heavy accent, and makes lots of grammatical errors -- but she doesn't let that stop her from having a good job and a rich life here in the US. I know a dude from Poland who's lived here in the US only 3-4 years, and speaks with a high degree of precision/fluency.

I think in the end it's very hard to generalize, and depends on lots of different factors (including what one's native language is, and how far/close that language is in relation to English). My own belief is that a determined adult who really works hard can often reach a reasonable level of English fluency in a year or so (after starting the study of language) if he/she is immersed in an English-speaking environment, with a high level of polished, precise English following in an additional eighteen months or so. For kids, the process seems to happen more quickly.

I don't know exactly period to learn Arabic .. may be one year or two  .. but i think there is no general base .. If you have motivation to learn it , you'll learn it quickly .. And sorry for the delay in the response.
My English is weak you know .. but I'll try to learn more and more to be perfect in English at the end of this year :)  

You're welcome. And what about Arabic -- how long does it take to learn?

Thanks peter ... I benefited from these values words .