Depends how many fish u have in there. If u have many fishes or if they are big in size, then they will tend to use up more oxygen. Well mine works ok. Just get those AC DC rechargeable one will sufficient.

My mom shut down the power supply accidentally quite some time ago. No cassualty though the fish were like out of breath. 6-7hrs without aeration.
The bigger the tank, the longer the time it takes to run out of oxygen...2ft...i have no idea...havent tried that...hehe