How much should I charge for web site ma

I have presently developed a web site for a compant in upstate N.Y. They have asked me to continue to support this web site. The web site is approximately 50 pages and requires that it be updated every three months and possibly some modifications inbetween those times.
What should I charge for this service? As a maintenance agreement what should I charge per month or should I charge a flat hourly rate only when I do work for them?