If you aerate with AIR, (an aquarium pump), you will hit a max DO (dissolved oxygen level) of 8 mg/L at around 20C. Your wort will STAY at 8 mg/L DO forever, since that is where it is equilibrium with the air that you were pumping through it. You will not lose oxygen if you let it sit for one hour or one year.
IF, however, you aerate with pure oxygen, you will get a max DO of something around 30 mg/L. Since the wort is exposed to air, however, this DO level will drop back down to 8 mg/L, (following first order decay), over a period of time. You still probably have a good couple of hours before it drops down too much, however, (I don't feel like calculating out mass transfer coefficients, etc....it's a lot of guesswork on my part anyway if I did).
IF you aerate with oxygen, and hit your max DO of 30 mg/L, then CAP the carboy, (so there is a blanket of pure O2 sitting on top of the wort), then it will stay at 30 mg/L forever....(as long as you cap it airtight). Realistically, however, you'll have leakage, and once N2 starts leaking back in from the atmosphere, your partial pressure of O2 in the headspace will drop, and O2 will start to diffuse back out of the wort.
Hope that's clear.
Edit: Note that the reason your wort is at less than 8 mg/L DO to begin with is because you boiled it. Oxygen isn't as soluble in hot water, (down to a solubility of 0 mg/L at 100 C), so your hot wort loses all it's oxygen. It's worth noting, however, that if you left cooled wort at room temperature and exposed to air, over a period of time, it would come back to 8 mg/L DO on it's own as it returned to equilibrium with the air around it. Of course this would take a while, and expose your wort to infection from wild crap in the air. The aquarium pump speeds up the process by greatly increasing contact area between air and wort, but it does not force "extra" oxygen into the wort, (you can never go above equilibrium dissolved oxygen concentration in the wort using air under standard atmospheric pressure).
