What is the lifespan for items stored in ViewState?

Answer 1

Items stored in the ViewState exist for the life of the current page.
This includes postbacks (to the same page).

Answer 2

It maintains the controls status during the round trip.
Example take yahoo registration you fill the all the details you missed one mandatory field after that you click the submit button. Registration it will retain all the control data. You have it enter what ever you missed mandatory data. If you will not maintain view state. You have to enter all the details
view state life time is with the page only. If you move the another page view state will expire.