One thing that I just remembered… the battery capacities are often measured at the 10-hour rate.

So a 1000mAh battery gets that rating because it is said to be able to supply 100mA for 10 hours. That often means if you try to drain 1A our of it, it might make it to an hour, but in probability, will start to drop off after 40 minutes or so, but could possibly run a 50mA device for over a day.

Take for example these three:

2.2 Amp Hour 12 Volt Sealed Lead Acid Battery | SLA/GEL Rechargeable | Storage/Batteries | Power Products Electrical | PRODUCTS | SB2482 | Jaycar Electronics
7.2 Amp Hour 12V Sealed Lead Acid Battery | SLA/GEL Rechargeable | Storage/Batteries | Power Products Electrical | PRODUCTS | SB2486 | Jaycar Electronics
12V 9Ah SLA Battery | SLA/GEL Rechargeable | Storage/Batteries | Power Products Electrical | PRODUCTS | SB2487 | Jaycar Electronics

So your mileage will vary. The harder you work a battery, the less time it will last. So my 30 minute estimate for the 1000mA pack might be on the optimistic side, it might be closer to about 25 or 20 minutes. The 20Ah pack though, if they've done their testing and design right, I'd be pretty confident should get very close to the 10 hours I mentioned earlier.