A1. You probably want to implement some form of merge-sort.
A2: Longer than it would if you had 256GB RAM on your machine.
Edit: stung by criticism, I quote from Wikipedia’s article on merge sort:
Merge sort is so inherently sequential that it is practical to run it using slow tape drives as input and output devices. It requires very
little memory, and the memory required does not depend on the number
of data elements.For the same reason it is also useful for sorting data on disk that is
too large to fit entirely into primary memory. On tape drives that can
run both backwards and forwards, merge passes can be run in both
directions, avoiding rewind time.