How much is generally taken off during alarplasty?

There is no "recommended" amount of reduction, because there are many individual factors that go into determining the "right" amount of reduction.  When a surgeon recommends 2 mm, she is referring to each side.  2 mm is a fairly conservative reduction, which may be appropriate in your case.  The final result is nearly immediate with isolated alar base reduction, but it will take a few weeks for the incision to fully heal and look its best.  

There is no set amount that needs to be removed.  It all depends on the current width of your nose, whether your nose flares with smiling, and what will still maintain a natural appearance with the rest of your face.  Generally, the recommendation of "x mm" will refer to each side.  The result is generally noticeable once the swelling goes does and the local anesthetic wears off.  Please consult with a board certified specialist who can assist you in achieving the results you seek.