How long does it take for nitric oxide to turn into dinitrogen and dioxygen?
Theoretically, it will decompose, but the reaction is rather slow, on a human timescale. One experiment ran for almost forty years (J. Phys. Chem. 1958, 62 (3), 360–361), from 1918 to 1957, without noticeable decomposition.

That said, in aqueous solutions it breaks down more rapidly. In vivo, the half-life is a few seconds.

It is amazing to me that a free radical is so stable!