> 50kb/s is too less if you have >1mb connexion. If
> select "no limit" , it download with 5okb or less. I
> can't wait 2 day or more to download >1gb 

First read <http://www.httrack.com/html/abuse.html#USERS> 

Trying to suck down more than that could result in a DOS on small sites.  As
long as you're not also raising the connection limit, then fine.

You aren't limited to the values in the list, just enter a value you want.