running time estimate of -lm

[Alexy Khrabrov]

I've launched ngram-count -order 2 -lm with a 1 billion word corpus a  
few days ago, and it's still going, after 4,600 minutes of CPU time  
(2.66 GHz Xeon 64-bit).  Originally it took about 8 GB of RAM, then  
decreased by about 25%, now is climbing back.  What is the overall  
running time estimate of -lm without any other options?  Simple runs  
for about 15 million words finished in about 15 minutes.

Cheers,
Alexy