How do you say when is dinner in greek?

The most straightforward way to ask "When is dinner?" in Greek is:

Πότε είναι το δείπνο; (Pote einai to deipno?)

This uses the formal word for dinner, "δείπνο" (deipno). You could also use the more informal:

Πότε θα φάμε; (Pote tha fame?) This translates more literally to "When will we eat?" and is perfectly acceptable in most situations.