R:"The Khumba Mela takes place every three years rotating through Prayag, Nasik, Haridwar and Ujjain." The next big one in Haridwar will be in 2010. It takes 12 years to come back to the same place (although there is also a half Kumbha mela every six years). In reality the big ones happen every 3 years since there are 4 cities that are divided in 12 years. If you add the half Kumbha melas, then they happen every 1 2/1 years.
R: The main one (very very big) is in Prayag (Allahabad near Varanasi) and it took place in 2001. The ones in other cities are big but nowhere near as the one in Prayag. About 75 million people attended the one in Prayag in 2001 compared to around 5 million in Haridwar in 1998.