Mass-marketed, national brands like yoplait generally have added stabilizers that lessen the benefit of the live cultures. If you can get a brand like Stonyfield (even though I think that is marketed just about nationally now), Brown Cow, Seven Stars - often found only in health food stores - you might notice more impact.
Generally, I eat about a cup, five days a week. When my systems seems to need it, I eat more (hardly even less). And a couple of times when things have really seemed out of whack, I have eaten up to a quart at a time.