August, as previous poster has said, is unbearable because the place is swamped by hordes of visiting boats. July is busy as well. Both months can also be very hot (and will be VERY expensive). While I was based in the Med
I would leave the boat
for about 6 weeks from mid July.
This area is sadly an illustration of mankind destroying its world. Yes, boaters need a permit
and there are many "protected" areas. However, you will rarely see any mooring balls but you will see tens of boats anchored in "protected" areas. One Sunday in July last year there were at least two hundred boats in the gorgeous "protected" area south of Isola Santa Maria, all anchored and only a handful of mooring balls to be found, at least one unoccupied! In fact, I have often seen boats anchoring
near vacant mooring balls. (One reason being a distrust of the integrity of the lines, another is that some balls have protruding screws which can damage a boat's nice shiny hull
In this peak period marinas raise their already eye-watering prices, rarely less than 150 euros a night for a modest size of boat
and two or three times that would not be uncommon.
It is not just the number of boats around these popular places at this time that's a problem, it's the people in them. Many use their boats (or charter) only in August and they bash and crash into berths and even into anchored vessels. There are party boats with thumping music
and shouting holiday-makers. We were surrounded by about 20 boats, mostly cats, one day last July in a bay in Sardegna immediately south of the Maddelenas, from an operation called "Bucket Lusters", which was basically a drunken orgy.
That all said, there are absolutely amazingly beautiful places in these islands and I would willingly go again. Just not in the six or so peak weeks. And not just the Maddelenas, all the way round the top of Sardegna to Bosa (good marina and boatyard) and the South Coast of La Corse (Bonifacio is not to be missed) are truly lovely places to sail.
Mistrals are quite common and the sea gets up really fast into nasty short chop of over 2 metres in anything over 25 knots. Forecasts of big wind events
are good, but not so for small systems which can still give you 25 knots when you don't want it.
Go! See for yourself and enjoy it (all the more if you bear in mind the above!)