Unfortunately it can take ages... I've had this problem with my phone. Some of the software allows you to transfer them in bulk though. (I use iSync myself now, but beforehand it was a bloody nightmare.)
It looks like at least one of the apps at that URL above does the whole lot at once.
I've suddenly realised, despite my almost luddite attitude to mobilated telephonic apparatus, I want one o' them t68is. Dumps to hard disk. How cool is that. :-)
Fwiw if you go from a T68i to a more modern Sony Ericsson, you should be able to send the lot as one big vcard, and accept it with one click on the new phone.