This study examines the relationship between the length of time since a patient received their first treatment for cancer and the feedback they give on that treatment. The analysis is based on a corpus of 214,340 written patient comments (14,403,694 words) relating to National Health Service (NHS) cancer care in England between 2015 and 2018. Patients are grouped according to treatment duration and compared in terms of the feedback ratings they gave, the themes of their positive and negative comments, and the keywords that characterise the language they use to provide qualitative evaluations of their experiences. We find that patient satisfaction rates remain relatively stable across the treatment duration groups, with patients generally indicating high levels of satisfaction. Patients focus on a small number of core themes in their feedback, regardless of treatment duration, though there are differences in the extent to which certain themes are focused on by each group. Our analysis of keywords reveals differences in the focus of the comments, as well as in the types of rhetorical strategies that patients (particularly those receiving treatment for longer periods) use, not only to frame but also to legitimise their feedback. The implications of the findings are discussed with respect to healthcare provision and feedback monitoring. We also discuss the value of combining a focus on content with form when working with non-linguist stakeholders, and reflect on the opportunities and challenges of convincing stakeholders of the value of using (corpus) linguistic methods to approach seemingly non-linguistic questions.