Battery capacity is rated in Ah (Amps x hours) or mAh (mA x hours). To find out the rating of your battery, look it up at the manufacture's web site. For example, look here
P = IE or I =P/E
If your camera requires 400mW at 9V, that means the current is I = P/E = 0.4/9 = 44mA.
At 44mA, the Energizer 522 I referenced above will have a capacity of ~500mAh, so would run your camera for 500mAh/44mA = 11.36h. Note that at the 11h point, the battery voltage will much less than 9V...
I bought a battery 12V 2.8 Ah - Lead Acid. I also purchase a charger. I had to charge the battery for 12 hours to increase the Amps. The battery before charge had a voltage of 12V. Why did I have to charge the battery when the battery was already charged?
The charger has the below information which you can also explain please.
Lead Acid (SLA) batteries have a fairly flat discharge curve. It is difficult to measure the state of charge by measuring the terminal voltage. The maker ships the battery uncharged, and expects you to charge it before putting it in service.
Your charger is capable of charging the battery to 13.8V, at a peak charge current of ~1A. It will only take that charger about 3-4h to bring a 2.8Ah SLA to full charge.