“Let us address the contention that it is virtually impossible to faithfully observe the laws of shmiras haloshon for more than a day or two:

Even if this were correct, is it reason enough to ignore this mitzvah? Imagine a person walking along the seashore, who sees that the sea has washed ashore precious gems. Would such a person — even if he were wealthy — refrain from picking up any gems because he knows it will be impossible to gather them all?

It is exactly the same regarding shmiras haloshon. It is well known that the Vilna Gaon (in his famous letter) quotes the Midrash which says that for each moment in which a person refrains from speaking the forbidden, he merits a hidden light that no angel can fathom. Note that the Midrash does not speak of refraining from forbidden speech for a month, a week, or an hour — but for only a moment!”

~ Daily Lessons from the Chofetz Chaim