I guess personal experiences vary. I often work in dusty environments (archeological and paleontological digs, as well as generally dusty environments like deserts) and I find the N95 very effective - dust is on the outside of the mask and the interior is quite clean. One day of sustained use is enough for each mask as a general rule. For these disposable masks there are a couple of simple steps to take to insure a good seal around the face (facial hair is usually a no-no).

This is one of those things that is worth checking out on an individual basis since YMMV.