How long is 3% hydrogen peroxide good for?

Okay. I am drawing a blank on this one. Had a friend ask me how long 3% hydrogen peroxide is good for once the bottle is opening. I know it lasts a lot longer if you keep it in the dark bottle it comes in. However, I do not know how long it lasts. The friend is a non-medical person that is curious. Anyone have any idea? Thanks in advance
hydrogen peroxide ......the average shelf life of a bottle is about a year but once you open it and use it, the potency decreases slightly. that's not to say that it will be any less effective in cleaning out wounds or for gargling - but it does lose its potency over time so its important to check the expiration date and if possible replace a bottle sixty days after its been open so you'll get the best results. ..www.epinions.com/content_122796609156 - 36k

We were just taught that for NCLEX purposes, in the hospital setting it is disposed of after just four days. I question that, but as a lowly student, who am I?