Of the two web sites I've done on the side, I haven't really had to face a maintenance situation where the client would need frequent updates to the site. But I need to be prepared when that does happen and I'd like suggestions on charging. The easiest thing would be to charge hourly. But it seems like a monthly fee might work to your advantage. Some months there may be little to do, while others might be a bit more intense. But if the fee were right, you'd still come out ahead. Any suggestions on monthly fees, or other options?
A company I work for gets $1000 a month from a car dealership. This covers database updates (must be done manually, they just send an email everytime they add or subtract a car) advertising and submission on directories, ffas, search engines, minor site changes, etc.
When we do maintenance, we usually charge an hourly fee of around $25-30/hr or fraction thereof. I find that this works well, as it can accurately measure the amount of time you put in to maintaining a site.
I think invoicing the company for the amount of time you spend is the best idea, but some are going to prefer a flat monthly charge just as some like to know in advance exactly what they are paying for the original design.
I think it all depends...billing hourly is safer for you, but if you have a good relationship with the client and trust them to not overload you with a bunch of updates (or if you're sure they can't possibly have much to update with), then it might be to your advantage to charge monthly...