Long-ago AES data was 17 dB peak-to-average for music, a factor of 50 (i.e. for music you would need 50 times the average power). Since average power may only be a Watt or a few, that's not all that hard, except with low-sensitivity speakers far away.
I have read here and elsewhere 20 to 30 dB for movies depending upon encoding and the mix (always). 20 dB is a factor of 100, 30 dB is a factor of 1000. That's a lot of power, probably more than most of us have on tap and more than most speakers can handle. However, such peaks tend to be short, and we are not as sensitive to distortion on very loud peaks (I am usually too busy covering my ears). I think based on no more than my own experience having 20 dB headroom suffices for me and requires perhaps 100 W to 300 W depending upon the speaker and room. YMMV - Don
"After silence, that which best expresses the inexpressible, is music" - Aldous Huxley