According to this link, on domestic flights the meal windows in the main cabin are:

• Breakfast: 4 a.m. – 9 a.m.
• Lunch: Noon – 1 p.m.
• Dinner: 5:30 p.m. – 7 p.m

In First, they're:

• Breakfast: 4 a.m. – 9 a.m.
• Lunch: Noon – 1:30 p.m.
• Dinner: 5:30 p.m. – 7:30 p.m

The list of requirements for a flight to even have meal service are listed here.

Hope this helps.