They charge for every use, and charge for not using it. And I have just been told that "loading" it does not count as activity, so does not reset the clock on non-use. It had previously. And they shortened the time range, too. It used to be 90 days, and then went to 60. And they still only sent an alert on the 90th day, at the same time they deduct the charge (for the second time), instead of, you know, a few days warning/reminder before the first one. (and if you don't use it in between, they deduct every month without notice,) I actually got a refund once, but they manage to delay the charge for a few days, so you might want to aim for 55 instead of 60 just in case.
Recommendation: Try elsewhere!