Hellene, thank you, great idea! I'm in too!
To give a chance to any world time zone from 9 AM-12 PM would be the best time interval. This way, even Hong Kong/ monlingling can be there if she can on Sundays.
If maximum 16 people are seriously interested than 1 call a month for each will be fine, no one will be upset, on the contrary

If more than that are interested than I suggest that we group ourselves according to more or less common things: time zone, age group, level of learning, non of these or any other reason
9 a.m or 10 a.m (your time zone) is possible for me too. It's after the kids take their nap and I can live them with someone to take care of them while being with you on Skype
Andrea