popular belief says fuerteventura is the windiest but they all get it, the wind will travel over certain areas 'acceleration' zones so take your pick.
Iv got a place in fuerte and had it 11 years, i feel iv noticed the difference over time and got chatting to a local who commented on how it was pretty much 100% guaranteed during the summer months back in the day but tend to see spurts of wind for 'x' amount of days then lulls nowadays so you can get a turkey during the summer months..that being said locals rarely own larger than a 5m or so so as a lake sailor my definition of wind is different to theirs

i have a good friend out there who runs a surf shop and he'l literally tell you what months do what to a startling level of accuracy (which is rather different to the wind stats you find on the web) anyway i generally go 2-3 times a year and october is my favourite for a hol but as my mate says its the least windiest of all and i would agree..that doesn't mean you won't, its simply luck of the draw, how long? obviously the longer the stay the more chances, i generally go 10 days and out there 28th oct to 8th nov,
other thing to note the trade summer winds are the N NNE the winter starts kicking in southerlys which turns the sotaventos and matas blancas into bump and jump from the flat stuff..really depends on what conditions you like and fuerte can offer all sorts if your prepared to ramble, i like speed and generally blasting about - il see if i can upload it iv got a quick vid of a run made in october last year to show it does get it (about 4 days of good blasting out of 10..would of been more if i didn't have a mrs)