By month and allocation of tasks. If you charge by keyword ranking, you won't have a guarantee that you will make money, as it is easy for SEO to fluctuate based on the conditions of the time and updates that happen as well as the dynamics of the website.

If you charge by month, identify which tasks that you have completed and offer to add in audits to showcase how the keyword rankings are changing. SEO is more than content, so add in different types of services within SEO, such as internal / external linking and how much, optimisation of content, technical services for the website. Define how much you want to charge with each of these.

I've worked with SEO clients for over 12 years and can help you too build your business as well!