Generics must work in the same length of time that brand name drugs do, per my reading of the FDA page.
My personal experience with generics has been very positive, I have never met a generic I did not like, lol. I do avoid taking any drug if a generic is not available. I prefer older drugs, especially since the fast track has been available at the FDA. I feel drugs are approved by the FDA altogether too fast, before enough long term studies have been done. This is why, I feel, so many drugs are pulled from the market after being approved by the FDA.
I have taken many different brands, and have never found any difference. Read the FDA guidelines for yourself at their website. I only take generics, not the original Oxycodone. I only use it for breakthrough pain.