According to the Indian government, the Kumbha Mela in Haridwar will be held in March and April 2010. That is the official story.
But the Kumbha Mela really starts with the "Makara Samkranti" festival (when the sun enters Capricorn, which will be on January 14 in 2010), when millions of pilgrims and sadhus start arriving in the city.
The Khumbha Mela officially finishes on the most sacred bathing day which will be on April 14, 2010.
So the Kumbha Mela really lasts at least three months (from mid-January to mid-April).