Text (7th+errata): Whenever an opponent casts a white spell, that player loses 1 life and you gain 1 life. [Oracle 2009/07/08]
The ability triggers when the spell is announced. This is before any responses can be announced. It is long before the announced spell resolves. [DeLaney 1998/10/05] For example, casting Disenchant will trigger this and cause the life loss/gain before Disenchant can resolve.
Loss of life cannot be prevented by any means. [DeLaney 1998/10/05]
