We did some of our furthest ranging cruising this past summer, and put on close to 800 nm (too many hours on engine, not a lot of wind and/or stiff current or tidal windows).
That has to do with the sailboat performance but mostly, like you say, with the wind or lack of it.
This year part of the season was on the Cyclades. We have been sailing there about a month in August. On that month I used about 60L of diesel sailing almost everyday between 20 and 60nm, maybe an average of 30Nm a day. I can only remember one time at a quay with electricity to charge the batteries so the Diesel went practically all for charging batteries.
I will recommend the Cyclades to all that want to sail and keep motoring to a minimum. Some experience is required because sometimes the wind is too much (4 days at anchor waiting it to blow away) and some smart navigation is needed to get away from stronger winds, but it is a delight to know that you will have wind to sail everyday and every night. I become a fan
On the other way, from Messina strait to Rome we only had a really decent sailing day, from Ponza to Rome (70Nm), but what a day, surfing at 11K with the waves directly on the transom