What I do...
For some clients I provide ad hoc writing services as and when required. I might produce a case study when a project has come to completion, or write the text for an occasional brochure or campaign. Other clients require more regular writing and therefore secure my copywriting services for a few days a month. I tend to work with my marketing clients for an agreed number of days a week or month, either in their offices or remotely. It all depends on your requirements and how much capacity I've got.