I don't think there is an average time a flare lasts. I have known some people who have the disease for 20 years and never been in remission and others who have flares that only last a couple of weeks. I had about 10 years of remission followed by a flare that lasted 2.5 years. The disease is unpredictable.
dx proctitis in 1987
dx UC in 1991, was stable until 1998
1998 started prednisone, asacol, pentasa, nortriptylene, ativan, 6MP, rowasa enemas and suppositories, hydrocortisone enemas, tried the SCD diet, being a vegetarian, omega 3s, flax, pranic healing, yoga, acupuncture, probiotics
2000 lost all my B-12 stores and became anemic
2001 opted for j-pouch surgery- now living life med-free