The Shulchan Aruch discusses this issue (Even HaEzer 76).
In Seif 3, he comments regarding the standard onot as fixed by profession:
בד"א, במי שגופו בריא ויכול לקיים העונה הקצובה לו, אבל מי שאינו בריא אינו חייב אלא לפי מה שאומדין אותו שיכול לקיים.
In what situation do [the above times] apply? For someone who's body is healthy and is able to perform his set onah. But for someone who is not healthy, he is only obligated according to what he is evaluated to be able to perform.
But note that in Seif 11 he writes:
ואם חלה או תשש כחו ואינו יכול לבעול, ימתין ששה חדשים עד שיבריא, שאין לך עונה גדולה מזו, אחר כך, או יטול ממנה רשות או יוציא ויתן כתובה
And if he became sick or weak and he is unable to perform his onah requiremnets, he can wait up to six months to get better, for that is the longest standard onah [that of a sailor]; then, he must either ask her permission to delay further [which is in general permissable] or he must divorce her and pay her ketubah.
So in summary, one may delay his onah requirements due to illness up to six months. If he still is too sick, then his wife may grant him more time or sue for divorce.