@debbielim
I feel like I was put around B1. Like SanneT said, in combination with other resources, you surely could.
Iād say Assimil followed by a couple dozen hours of speaking practice will get you to B2.
The lessons speed up greatly after week 2 of the course. For all the assimil courses that I own I have had a native speaker rerecord the first lessons at a natural speed.
@David
I guess it also depends on the exact language and the learnerās linguistic background?
In my case, if I were to spend 6 months doing Assimil (or similar) for Spanish, Iām pretty confident that Iād be at B2-ish level. But Iām a native speaker of English and I already know another Romance language (Italian) to a pretty decent level. If I were trying to get into Arabic or Japanese, Iām certain it would be another matter entirely!
BTW
I think that learning a language which is quite closely related to one we already know brings its own unique set of challenges. If I wanted to get into Dutch or Afrikaans, itād be - at one level - relatively easy for me due to similarity with German and English. Yet Iād probably have to be super-motivated to do it. For example: looking at Deon Meyer texts Afrikaans is really slightly weird; as foreign languages go itās pretty transparent, yet it can also feel strangely āwrongā in comparison to the English and German which is more dominant in my brain. And there is, of course, a significant difference between passively understanding something and being able to produce it in an active wayā¦
@Jay
I was directing my comment to the poster who said that heās a āfalse beginnerā in German, so I completely agree with you 
Iād bet that you can still get a pretty strong passive level this way in a language like Japanese, but I havenāt done it yet ;p
The way I calculated it in my head when I posted was like this:
100 lessons X 30 minutes each = 50 hours
+
At least two hours a day of review (listening to the lessons on loop while doing other things 100 X 2 = 200) = 250 hours
+
24 hours of speaking lessons = 274 hours
For fun letās add in a month in country.
If the studentās high school or college class was a year or two long, and if he actually did his homework, the combined number of hours would easily surpass those necessary for a B2 level.
Thanks everyone for the posts. I thought Assimilās claims within their time frame were pretty impressive so I was a bit sceptical. But it seems they really are realistic, if Iām a native speaker of English learning German, so Iām pretty excited. Schauen wir malā¦