[John Locke]

The answer is "never". In the Moslem calendar, the month always begins with the sighting of the new moon. So the first night of Ramadan will always fall on a new moon, when a lunar eclipse is impossible.

By the way, almost all Moslems deny the authenticity of this hadith. It was first recorded by Amr bin Shamir, who was a notorious liar, and his attribution of the saying to "Mohammed bin Ali", ie Imam Baqir, is almost certainly fraudulent.