I have owned many loudspeakers. Most required minimal break-in. Occasionally a few hours were needed. The worst example was my Dahlquist Preludes, which remained edgy and rough sounding in the upper midrange for many days. On the verge of being returned to the dealer, they finally smoothed out after more than 100 hours of break-in (in a closed room at moderate volume). There is no single rule for all loudspeakers.
Here are recommendations from Paradigm and Definitive Technology, respectively:
"Although Paradigm Reference speakers sound great 'out of the carton,' they will sound even better once they are 'broken in.' Allow them to play for several hours before you begin any critical listening." (OM-1000 user manual, page 2.)
"Your Mythos ST-L Supertowers should sound good right out of the box; in fact, there is no specific break-in period. The best recommendation we can make is to play your speakers. We have found that after around 40-60 hours (or more) of normal playing, the suspensions work in, and this results in fuller bass, a more open 'blossoming' midrange and smoother high frequency reproduction." (User manual, p. 19.)
Music room: McIntosh MA6500 integrated amplifier, Cary SLI-80 tube integrated amplifier, Quad 99 preamp, Quad 909 power amp, Acoustic Research AR9 loudspeakers, Yamaha CD-N500 CD player, Teac UD-503 DSD DAC, Phase Linear 8000II linear-tracking turntable.
Theater room: Panasonic TC-P65S60 plasma television; Yamaha Aventage RX-A2020 (preamp section); Adcom GFA-5503 and GFA-5400 amplifiers; Polk LSi25, LSiC, and LSiF/X loudspeaker system; Velodyne FSR-18 servo-subwoofer.