Hi Simon,
I very much agree with Mark and Oleksii the estimation of time can never be very
accurate because think if another user take a logical lock on a page or row which this statement want to update now everything depend on this other user and when he release the look this is
not even always up to the RDBMS.
But if this is some batch related sql, I would collect statistics for its previous runs
and work out my estimate from that.
Rgds, Niclas