It takes me about 10 minutes to process the whole of GB so something must be wrong with your setup.

I have though had a couple of cases recently where mkgmap has just got stuck and as the it was still consuming resources it looks like an infinite loop. In the end I traced the reason to problems in my style files - the latest problem disappeared when I removed the comments without touching the code itself. It looks to me as if they are tightening up the style processor and that if something goes wrong there is chance of an infinite loop rather than a crash.