Magic Music duration

Hey, I noticed that magical music doesn't always last as long as the whole song, for some VERY long songs. For example, I was playing a very long silent song with +str. I checked back some time later, an hour maybe, and I no longer had the strength boost, or the "magic pulse" appearing. Does anyone know what the time limit for the music is? It would help me, as I plan on afking as a mana totem to help people make adv crystals.