"What time does she want dinner?"
Translation:Am faint o'r gloch mae hi eisiau cinio?
It depends on what the question is about.
Questions about how, when, where, why, how often, how quickly, etc, or which start with a preposition or an adverb are generally followed by mae - sut (how), pryd, ble/lle, pam, pa mor aml, pa mor gylfym, am beth, i bwy, gyda phwy, etc.
Am faint o'r gloch...? is asking when, so it is followed by mae.
- Sut mae Dewi heddiw? - How is Dewi today?
- Pa mor aml mae e'n dod yma? - How often does he come here?
- Ble mae e'n eistedd? - Where does he sit?
- Gyda phwy mae e'n eistedd? - With whom does he sit?
- Â phwy mae e'n siarad? - Who is he speaking to? (With whom is he speaking?)
- I bwy mae'r anrheg? - Who is the present for? (For whom is the present?)