What happens when auto-pilot fails over the ocean?
As @Bianfable points out in the comments RVSM airspace is a problem but ignoring that for a minute: 

The question of will they continue? depends on the airlines op-specs. Different airlines will handle this differently and it may very well be handled different on different airframes within an airline. A call to base may even occur but there are lots of factors that can play into it such as what other equipment is operable.  

Assuming the Op-Specs dictate that the flight continues


  how difficult it may be for two pilots to manually fly for this long


Potentially not too difficult at all, most pilots trained in aircraft with no autopilot and all pilots are qualified to hand fly the aircraft. However after Op-Specs it is ultimately up to the pilot and they will consider many factors. Hand flying an aircraft in IMC can be a tiring task for even the best of pilots, a pilot may chose to divert or land sooner if they feel it would be unsafe to continue even if they are capable of flying the aircraft by hand.