When will the Kalki avatar of Lord Vishnu take place? What importance will Sambhal town have in this avatar?

 This is one of the major misconceptions by common Hindus and language of Puranas pushes the simple minds to such beliefs. In fact, the word Kalki is not a noun — that is, not the name of a person — but a prepositional phrase. It originates from Kali-ki, meaning “for Kali.” The word itself conveys hope — that there will be an avatar for Kali, or for the darker times.

With that understanding, the Kalki Avatar is not necessarily an event that will occur as an end-time prophecy in the lifetime of this planet earth, as many assume. Buddha could be considered a Kali-ki avatar, since he was indeed born during the Kali Yuga, he was divine and carried sword of wisdom, riding on the vehicle(path) of righteousness.

Moreover, the concept of Yuga itself is quite complex, and leads to another major misconception in Hinduism.

We are no more in Kali Yuga on the time line of earthly humans. We are actually in dvapara Yuga. (and may be in Kali Yuga in galactic time line.) So stop believing end time prophecy and start living in current reality.