From todays press conference, the estimated fuel after the last ping was 30 minutes....
They did not explain how they came to that number considering they do not known the altitude or speed of the track it took to get to the last ping point.
I believe it is an estimate & it could also be an average since the shortest time after the last ping to flight end would be 1 minute & the longest 59 minutes.*
* assuming 1hr pings (has this been confirmed?)