Hello,
It is quite a hard question to give any direct answer to. Speaking of own experience I would suggest that you spend good time on planning the sharepoint environment and how you will use sharepoint as your Intranet. The more time you spend planning the usage, the easier the migration and maintenance will be.
Microsoft has publsihed several documents and white papers on planning and implementing SharePoint, which I found quite useful in the beginning.
I am working as 'Operation Manager' for our sharepoint environment, hosting several hundreds sites and around 7000 users. What I have done is to make sure that each team that has a site (or portal area) appoints atleast 2 persons that will work as Site Adminstrators and that they can take care of user administration, designing the sites (or areas), etc.
Once you have the sharepoint environment in place and you have 'executed' your plan on how SharePoint will be used, the maintenance will not be that much (but again that depends on the ammount of data, ammount of sites, if you are running sharepoint services or use the sharepoint portal server aswell).
Are you planning to use Microsoft SharePoint Portal Server or Microsoft SharePoint Services?
A book about Microsoft SharePoint Technologies that I found useful is: "Microsoft SharePoint Technologies - Planning, Design and Implementation", ISBN: 1-55558-301-6
Cheers,
Thomas