Couple of questions using these facts...

1 - The company I work for has a staff of 500-535 people.
2 - We have 3 sites, external website about 100 pages of mostly brochure pages, intranet which is about 500 pages of ASP, with only about 10% dynamic, the rest static brochure pages.
3 - An IT staff of about 15, 4 of them in the applications group. The applications group knows nothing of web based applications.
4 - We don't have a web developer, master or any other sort of official web person....
5 - The web duties are within the Marketing group mainly because they generally provide most of the content, but I've been doing it for 2 years now I'm a member of the IT group. My official duties are LAN administration with web duties assigned as about 40%. The work of the web would be nearly a 100% full time position.

Now the Questions:

1 - When is it time for a company to hire a webmaster, web developer or some other sort of web professional... (Or hire me in that role)

2 - Where typically does a web position 'sit' in the greater scheme of IT and Marketing?

I realize that each organization is different and have special arrangements but in a general sense...

Thanks for any comments..

Good questions, and the only answer that I can give is: it depends.

Given the size of your company though, I'm a little surprised you don't have a dedicated person (contractor or otherwise) working on the web site.

In my experience, it seems that companies who have "information only" web sites, can get by with far fewer resources (web programmers / admins / etc) than companies who derive some sort of income from their site. If you're running an eCommerce site, or other revenue generating scheme, you need someone to 1) keep your site up to date, and 2) make sure the site is running. Down time = money in those cases. You may not need someone "dedicated" full time to the site, and usually with smaller businesses, they have a marketing / web programmer position who keeps a close eye on the web site, but also does other things.

For a web site that is purely informational (which it sounds like yours is), changing content usually gets assigned to a staff person who knows dreamweaver / front page and maybe some basic HTML. If a brochure doesn't get posted on Monday, it's "usually" not critical (unless you have a mass mailing going out that references something that needs to be on the site, but you know what I'm getting at.).

As far as where the position goes in the structure of an organization, that's all over the board too, and I've found usually depends on what senior management thinks a web site is. If they think its "techy", its usually under the IT / Finance deparment. If they think it's more a marketing tool, it goes to Marketing. More often than not though, I find them to be in IT / Finance.

As far as when to make it a full time position, I'd say that if there's more than 75% of a FT equivilant workload for web development, then you need to create the position. IE, if you can say that you're consistantly working 30 hours / week on web only stuff, then you need a web developer position.