The ancient AES guidance I recall is 17 dB (a power factor of 50) for music based upon studies of live music long ago. That is, if you need 1 W average, expect to need 50 W to prevent peaks from clipping. More recent anecdotal data is more like 30 dB for movies, a factor of 1000, but I expect that is mostly for explosions and such where a little clipping will go unnoticed. For studio work, again my experience (pretty dated), the pro standard was +4 dBu with 24 dBu max so 20 dB headroom required (a power factor of 100).
From there the power you need depends upon how loudly you want to listen and at what frequency, the sensitivity and dispersion (radiation pattern) of your speakers, your distance from them, and the room's characteristics. And probably a few other things I've forgotten. And don't forget that if the numbers say 1000 W but that fries the voice coil or whatever in your speakers then having a 1000 W amp may not be your best choice.
FWIWFM - Don