I pay 3.85 cents/minute for regular, dial 1+ long distance from my home phone (either line -- nice and convenient). 12 hours of that comes to $27.72. Less calling comes to less money, obviously; 6 hours would come to half of that, or $13.86.
You may not want to pay $25/month for a fixed rate plan of any kind. Then again, with the freedom to call as much as you want, you may.
There are plenty of $20/month competitors to Vonage. Packet8 comes to mind; $20 for unlimited US service.