I don't know, to me a "nap" is a resting/sleeping period during the day when I'd normally be up doing stuff. So I don't base it on length of sleep so much, but more timing. When I'm sleeping overnight (for however long) that's sleeping to me. If I'm sick and I stay in bed all day dozing, that's sleeping to me. But on an otherwise normal day, if I lay down during the middle of it and doze off - that's a nap, however long it is.
I don't usually nap very long, and if I do it too late it will mess me up for sleeping that night. My man can take a LOOONG nap, a couple hours easy sometimes.