Brief Answer:
Nearly one month if treated properly.

Detailed Answer:
Hi XXXXX,

Thanks for posting your query here. I understand your query as well the concern regarding your health.

As per my opinion your history is not much informative, so I will through some light on the disease aspect.

Diverticulitis is a disease of the large intestine(colon) which is characterized by fullness of abdomen, bloating, flatulence, vague discomfort, sometimes pain etc

Considering your history as you want to know how long swelling will take to go down completely??

In my opinion It will take around 1-2 months if properly treated by high fiber diet, antibiotics, avoiding constipation, regular exercise, green leafy vegetables, plenty of oral fluid, yoga meditation etc.

I advice you to follow the instruction given by your treating doctor, and if possible update me with your other relevant investigations done.

Hope I have cleared your query. I will be eagerly waiting for the response from your side, don't hesitate to write me again.

Wishing you speedy recovery & good health.

Kind regards,

Thanks.

---