The ideal length of the chain has to do with the deep. No sense in having more than 11 times the deep and in most cases 7 times will be enough. On very strong winds the chain will be completely stretched so what matters is the angle of the chain with the anchor.
Some will consider it another way: deep of the water
plus height of the bow (around 1m)X4+2 times the LOA
of the boat in meters. That will give for a deep of 4.0m and a 12m meters boat: 44m of chain
I don't understand what the length of the boat has to do with it and I prefere in extreme circumstances and if possible to have close to 10 times the deep and that is only possible if you chose carefully the place where you will anchor to wait the blow to go away. The good news is that most prefer to do that at a Port or marina so most of anchorages
on those waters and those conditions will not have many boats on anchor.
On that area of the Cyclades you will find plenty of anchorages. Use this pilot book:Publications
and you will get plenty information.
Regarding my personal experience on the area on those months I had got in two months about 2 to 3 times where I prefered to stay at anchor for 2 to 3 days till the blow went away. I will prefer to stay at anchor when a F8 is forecasted and even with a F7 if the sailing I want to do is not downwind.
I have heard of longer blows (till a week) but I never got that in two years sailing there.