the 10 days thing comes from the MSR and FTR on adsl. But those arent on FTTC connections.
Also I think isps advise 10 day stabilisations as I am guessing they assume unstable lines will be put under control by DLM within 10 days, so they do it to stop customers ringing up complaining.
The reality is tho on FTTC speeds generally start as high as you will ever see them and from that point the only way is downwards. This can carry on happening well outside the 10 day period.
It does seem alot of things broadband related is about trying to just keep the end user mellow, engineers lieing to customers and so on. I had one trying to mislead me on friday when he visited to look at my line. Telling me my estimated speed is only possible if next door to cabinet. I of course tried to tell him my estimated speed would be the full speed if I was next to the cabinet and my line was under performing, he wasnt interested. He even diagnosed a fault and then decided he hadnt enough time to fix it even tho he was here well under alloted 2 hours. So expect to be fed a load of bs on FTTC from various fronts.