Hey Guys Love the Forum!

I'm a newbie to the web development field, and being as such I've got plenty of questions. I'm sure you guys will be instrumental in my progress.

Currently I'm having trouble figuring out how to handle site maintenance. I've only built simple sights so far so it hasn't been an issue, but I'm curious why is it that most sites end up requiring maintenance? I've built in basic cms processes into my sites to enable my clients to alter content, but I'm sure there are reasons other than content updates which demand maintenance and want to be prepared when discussing proposals in the future.

What types sites typically require what level of maintenance? How do you determine how frequently a site will need to be updated? When should I charge upfront, monthly or as needed? In general what have you found to be the best practices for handling site maintenance?

This topic is the biggest thing keeping me from soliciting more complex business. Any insights you may have would be greatly appreciated and I'd love to hear as many opinions as possible as I'm sure you all have different methods.

Thanks in advance!