We rely on Skype because of our overseas relatives in England. We have a 5 gig plan with ATT and it rarely uses more than 2 gigs a month. We had Netzero for a couple of years while we workcamped and even with a slow dialup connection Skype was useable, though it would fade in and out on occasion. In our experience it doesn't take much to make a Skype connection work. Never really paid much attention to usage per minute though.
I should add, using the park wifi here we don't have any problems with it and when on the road almost any wifi that is strong enough to get into our email accounts is strong enough to use Skype. And no, I don't have any affiliation with Skype, just a happy customer that saves a lot of moola using them,
One last thing, we average 6 hours on Skype per week.