the maximum real life transfer speed of FW800 should be around 50MB/s. With this speed it would take ~1,5 hrs to transfer your 250gb.
BUT this is for big files (sequential reading/writing). But the Aperture Library is a big folder with farely big files (your pictures) and lots of small files (your changes to the pictures, databases, etc). So the Harddrive needs a lot more time to transfer those.
19hrs for 250gb mean your drive transfers ~3,5MB/s. Thats not very good, but still realistic.
In short: be patient
I copied a 212Gb library from external drive to a new internal drive via Firewire 800 and had the same estimates. It actually took a bit less (about 15 hours from memory) ... but my issue was corrupted thumbnails when it got there.
I chose to do it all again (from another internal drive that I was swapping out rather than the external back-up) and cloned rather than copied - that worked.
Thanks guys for your responses.
It actually ending up only taking 3 hours! The estimated time went from 30+ hours to a little under 3 and finished close to that. Very happy with those speeds. haha It said the larger estimate for about 20 minutes before it began to change and I noticed it was working much faster, gradually almost.