If you ask "What time...", you are expecting an answer with a specific time, such as 2:30 PM.
If you ask "When is...", the answer could be more general, such as "early in the morning", "late in the afternoon"...
The four first sentences are correct, although I would prefer to ask: "What time is..."
The last two sentences are not correct. A better way to ask those questions would be: "When could I leave?" and "What time can I leave?"