If it appears fair to the writer, it's probably all that is needed. The conditions offered are still negotiable. An hourly rate is common for technical writers. The hourly fee acknowledges that the project involves unknowns. Often, it doesn't matter how long the work takes as long as the client gets what they want.
Most freelancers do care about hours.
They also want a good reference for the next job. This can be spelled out in a lengthy legal document or it can be a simple letter. A Gantt Chart is a good way to communicate all aspects of a project. A Gantt Chart shows each significant step of a project as a line on a time-based chart. Each step is shown as a horizontal line in sequence from top to bottom.