Sending them a reminder seems fair. And as dangrossman suggests, sending it on the 7th day is logical. But I would go one step further: take the time to ask the

Sending them a reminder seems fair. And as dangrossman suggests, sending it on the 7th day is logical. But I would go one step further: take the time to ask the user if they're liking the service and if they have any feedback. I would also remind them in that email that their credit card is going to be charged.