How long can I run my camera off this battery?
My approach to calculate it would much simpler than what Blue Sky did.

$$ E_{bat} = 12V * 7Ah = 84Wh $$

$$ t = 84Wh / 1.25W = 67.2h $$ 

Assuming 80% buck converter efficiency

$$ t = 67.2h * 0.8 = 53.76h $$