The performance loss of a good 20' extension will be just audible, in my experience, while a 15' of the same cable isn't. But I think it's mostly a measure of the cable; if you used very expensive cable you might get 20' without hearing it. Also likely depends on the output impedance of the amp - the higher the output impedence the more likely you are to hear problems.

Grado makes a nice headphone extension cable that offers the bare minimum of degradation (there is ALWAYS some loss.)