according to Dr Fuhrman MD concerning autoimmune disease in general and in particular with arthritis................
"Fasting is a remarkable anti-inflammatory intervention, more powerful than the strongest and most toxic drugs at reducing inflammation. It is a crucial management tool for the person suffering from autoimmunal disease, and, when combined with appropriate dietary modifications, can induce a remission of symptoms.....
A supervised fast is the most effective way the individual with Rheumatoid Arthritis can quickly eliminate pain and stop the degeneration of the body and joints caused by the attack of the immune system.
A definite benefit is obtained when all dietary antigens derived from foods are removed by fasting. When patients with Rheumatoid Arthritis are allowed only water on a seven-to ten-day fast, joint pains are substantially reduced and stiffness decreases.
Fasting also reduces inflammation by altering excessive activities attributed to the white blood cells and other components of the immune system".
in Fasting & Eating For Health.
But he does emphasize the vital importance of a plant-based diet as outlined in his book: Eat To Live.
I would also think that a 14 day water-only-fast would be very effective, but this improvement will be ongoing if you adhere to the diet as described above.