The OU guide to Kashrus in Israel says:
White flour: In the US there are special methods used in processing
flour to insure that it remains bug free until use. Unfortunately in
Eretz Yisrael flour is not processed in the same way and insect
infestation can occur. Therefore it is important always to sift flour
before using it. You may sift more flour than you need and freeze the
excess. This should be done immediately. Flour that is stored in a
freezer immediately after sifting requires no further cleaning.
A 70 mesh sieve is recommended. These are available in most kitchen or
Whole wheatflour: It is nearly impossible to sift whole wheat flour
with a 70 mesh sifter. While some poskim are lenient and allow a 50
mesh sieve to sift whole wheat flour, buying either frozen or vacuum
packed whole wheat flour with a reliable bug free hashgacha is
recommended. When buying frozen flour, check that the flour was kept
So the criterion is 70-mesh.