Take the 101 course, a good introduction. Then you might consider chartering with a skipper, worth the extra expense because he/she will inevitably teach you and you get to do as much of the sailing yourself as you want, with no other students, and you still get a vacation out of it. I suppose a good Basic Coastal Cruising course would do the same thing. Then find a friend with a boat and crew a lot.
I teach some, but I personally think that sailing is best learned over time, with different instructors (including the "school of hard knocks" on your own) and varying weather conditions, boat types, water temperatures, climates, and geography. Just getting the certificates may not be enough depth of experience.
Want a wild estimate, since you asked? Do the above, sail as much as you can, do some short-hop cruising, give yourself about two years (?), then try that cruise in Greece.
Caveat here: I'm a monohull guy, and don't think I can give advice about catamarans. I'm sure they're great, but I like upwind sailing and tacking quickly whenever I want, and I don't see most cats do this well, though they're fast on a reach or a run (cat fans, flame away).
Anyway that's my curbstone advice. It's free, and therefore worth every penny. Of course, your mileage may vary, but you did ask..