When. Morning and evening, within the standing set of daily remembrance. It is short enough to be said while walking, and it is often the one people fall back on when they cannot recall a longer text.
The words. Everything here turns on al-'afiyah, a word English cannot carry in one piece. Its root, '-f-w, is the root of pardon and of effacement — the same family as al-'Afuww, the One who erases faults. So 'afiyah is not simply good health. It is the condition of being spared: intact in body, unharmed in religion, untouched by the trial that could have come and did not. A person asking for 'afiyah is asking for the absence of something, which is why it is so easy to have and never notice.
The definite article matters too: not "some wellbeing" but al-'afiyah, the whole of it.
Both houses. Fid-dunya wal-akhirah refuses to split the request. It is easy to ask for relief now and easy to ask for salvation later; the supplication insists on asking in one breath, and puts the two on the same footing.
A longer form. The request is often extended — wellbeing in one's religion, worldly life, family and property — and that expansion shows what 'afiyah was already meant to cover. The short form is not a reduced version; it is the same request without the itemising.
Why ask for so little. It reads like a modest request beside asking for paradise or provision. It is not. Most of what wrecks a life is something that arrived, not something that failed to.