It's based on server time, so it ends at the same time for everyone. They'll announce what time the freeleech ends in advance; right now no one knows when exactly it will actually end. Server time is technically GMT but chances are the cutoff will be later than midnight GMT (think about how many Americans/Canadians use the site), so asking about time zone is moot.
But to clarify, the end time of freeleech WILL be announced at least a couple hours in advance; IIRC it always is.