Depends on what the original agreement was. Typically we include a certain amount of revisions in the original quote. Anything over would go hourly. If you charged a fee for the design without noting to the client revisions are extra- you might have an angry client. If they know ahead of time- your in the clear.
Also, we NEVER label things as "artwork." always use the term "layout." if you say artwork- the client thinks it's their property. If your anything like most shops, you do the package at a discounted rate in order to get future work. We invoice "layout" with client option to buy the "artwork" for an additional cost.