jonr-3 from Kansas City wonders if the 265-minute version will ever be released.
The answer is a definitive NO because every frame of unreleased footage no longer exists anywhere in any form.
It's a shame, because the film -- fascinating and electrifying as it is in its present form -- would probably have been one of the greatest examples of intertextual cinema of all time had it survived!
One can easily imagine what we're missing simply by examining the way that the initial scene on the train plays out as Mori explains his dream about nearly being executed to Mifune -- and then we are presented with a jarringly disturbing cut to a long intertitle, which basically seems to explain what was cut out by the studio execs [as do the many intertitles which follow]...
Kurosawa's hero-worship of Doestoevsky may be compared to his similar adoration of Gorky and his play "The Lower Depths" -- which is faithfully adapted in the 1957 filmic version -- and although it is much shorter than the tale told by The Idiot {sorry, couldn't resist!}, this reverence in no way makes the film boring or inferior. Just compare it to the 1936 Renoir version (which is quite good in many ways in its own right) to see how this faithfulness pays off...
Read the Doesty and then watch the film and fill in the blanks yourself. Kurosawa's filmic blueprint provides plenty of clues to how the missing footage might have been incorporated into this extremely underseen masterpiece.