How long it take to run in brand new loudspeaker?

I just bought a pair of QUAD 11L Bookshelf and found that the high is bit soft, is that due to insufficient run-in?
I think that it is usually the other way around. Speakers that haven't been broken in seem to be too bright / harsh high frequency characteristics. As they break in, the high frequencies should mellow out a bit. So, not sure about your situation. It may be that these speakers have the general characteristic of being soft in their high frequency response??????
my experience was just the opposite of Bufus - new out of the crate my speakers (not quads)were very rolled off - muffled in fact. i didn't count the hours but i let them run for 3 to 4 days and the opened up just fine. be patient.
It really depends on the speaker and how hard they're run in. 50 hours at a decent volume will make most speakers start to sound mature. Many speakers take a good 100-200 hours to really sound their best.
My experience was the same as Bufus and I have found that a few hundred hours are needed before reaching steady-state sound no matter what low number the manufacturer gives for break-in. You may just have bought what are laid back speakers to your ears. Arthur
Aren't there two issues here? Maybe some tweeters have flexible surrounds, and thus will increase in output (get brighter) with breakin; likewise, and more commonly, woofers and mids surrounds and spiders nead to flex a LOT to break in, thereby FILLING IN the bottom. Maybe some do BOTH, widening the "shoulders" spectrally, as it were?
About 500 to 1000 hours. But of course, the speakers will continue to sound better and better until they break.
For me it has always been the bass to clear up last.My Aerial cc3 sounded like there was a comforter over it for 3 days,or more. I actually put one on it and ran it loud for 4days continuously. Then there is the Korus. A dealer in a phone conversation, said this speaker improves till it has 3,000 hrs on it.
Depends a lot on the system, of course. The Paradigm 100s in my HT set up needed 300 hours to settle in. OTOH, the Mezzo Utopias in my music room didn't change much from day one.

Seems like the best performance speaker will only have after 10...15 years of home usage and after a couple of years of heavy commercial usage.
Please, rub your head... 1000hours????????
I've only run in two pairs of high end speakers Sonus Faber Concertinos and ProAc Response 1SCs, and in both cases they sounded on the thin, tizzy side brand new. After a month or more of play, the midrange was more open, the treble relaxed yet still detailed and overall more liquid. Also soundstaging grew and center focus locked in better.
I have just confirmed that It is due to insufficient run-in. After 90hrs of usage, the baby quad produce highs so beautifully that you cannot believe it come from a budget speakers!

I have just confirmed that It is due to insufficient run-in. After 90hrs of usage, the quad high is so beautiful that you cannot believe it come from a budget speakers!

Ahh, that's great to hear. Literally! I've found that some components can take much more than 90 hours to burn in sometimes over 500, so it's possible you might enjoy further improvement. Have fun.
I recently heard the new Monitor Audio Silver S line of speakers and they sounded fantastic right out of the box. I mean they were unpacking them before I listened to them.

There have been many threads on different forums on this exact topic and I wonder how much is psychology and how much is break in. On one forum somebody mentioned that he couldn't measure the break in, even though he heard a difference. If can't measure it, is it actually there?

botrytis, i think the only way you will know for sure is to buy a lot of speakers and break them in to see for yourself if there is a difference. Please do not trust the experiences of others - we lie as lot just to get in print.
Everything can be measured. We live in the world where physics dominate over voodoo and some established stereotype human beliefs including religion.

90% when you hear people speaking that the speaker sounds bright or dim in terms of higher freequencies out of the box is the matter of personal adaptation rather than break-in when it "starts to sound ok". The midrange problems might have some large loudspeakers that realy need its voice coil to be "massaged" with some low freequencies to make it more flexible along with its massive drivers. To the high quality speakers with 7" woofers the break-in time can be very minimal(from 10 minutes to few hours).
The main role to confuse and convince people takes an advertisement that brings up "professional reviews" that make people believe that after 1000 hours their speakers will start sound better(otherwise there will be too many returns). It's even possible to make people believe to many many different things beyond it especially when it comes to play with money.