How long would it take me to learn Hebrew if I already speak Arabic?

Anyone can become conversational in about any language with a years worth of hard study. Just because you know arabic doesn't mean you will attain fluency any faster than anyone else although you will be more accustom to certain sounds and that will be easier but there is still the writing system to be aware of. As far as fluency goes as long as you are actually living in israel it could take 2 years (because you will be forced to use it every day and what not i know a chinese girl who became fluent in english within the two years she had been in our high school no accent) but you will have to work TWICE as hard as before. So here is how i break it down. Conversational - 1 year Fluency - 2 - 3 years (working HARD and being immersed in the language) Fluency - 3 - 4 years (working at a steady pace in a country that does not speak hebrew)

If you intend to study grammar it will be easier cause its very similar, but spoken language depends on your will,age and ability to study new languages. I had arabic in school but I am not fluent in arabic at best I can read restaurant menus and do very basic conversations.I always paid special attention to cursing in arabic, I can tell when Arabs speak nasty stuff usually right away. For me arabic is easier than learning Russian or German....Living in U.S when I encounter obvious Arabs like vendors with "Ahmad" name tag ill speak to them in Arabic, they don't always respond positively to it.

The fact that you are an Arabic speaker, does not make it easier on you to learn Hebrew

I can tell you it took me two years to learn Arabic. I studied it in school, i still does but now after we went last year to the summer vacation i totally forgot everything. I used to get A's all the time and now i get C- >< So it will probably will take you two years if you'll learn 4 hours a week.

If you work it will take a year and a half, if you do not then a less than a year.

They do have similarities, but it's still going to take some work and effort, and most importantly time and practice. Of course, that goes with any language, but I'm just saying that, while the two have many cognates, that it's still not something that I would expect to easily pick up. Especially if you are in your adult years, in which case learning any language would be difficult. If you are good at learning languages, and have a good learning system, then I would say that it would take a year and a half for you to get to a level where you have a strong comprehension. A mastery or fluency of the language would take much longer. That's not taking any big breaks, even. Consider, though, that I'm factoring in other things like age, jobs, family, time for fun, and other commitments you might have.