Calculating Average Length of Treatment (Time Elapsed)

Hello Excel Experts!

I was wondering if I could get some help calculating the average time elapsed from a first session to last session of treatment. Where I've run into problems is figuring out how to do this when different patients have differing numbers of rows (dates of treatment). Per the attachment, I've specified whether a date is an initial, return, or final visit, which is based on sorting by patient ID. Is there a formula which can calculate the time difference b/w first and last session, but specific to the patient? Thank you in advance for any guidance!