It depends on your PC spec , coversion is very processor intensive , so a fast CPU is good here ....
I have a core 2 quad 6600 , an 8800 graphics card ( which I dont think makes any difference to this particular issue ) , 4GB ram , an LG bluray/HD DVD reader/writer , and it takes me about 20 minutes.
Thats for content recorded off the RF2 of the sky box , i.e. just SD composite stuff.