I use Zoner's HLvis, HLrad... and his other tools because of the speed and NULL textures.
Yeah... the VIS and RAD take long... but the VIS in particular.
If you have a lot of small objects, angles, or other complex non-box objects all over your map, your VIS compile time can take a very long time.
Also, screen-saver will screw things up. I always set mine to "turn-off-monitor" instead.
Anyways, I did such a modification to my map, and my compile time went from 1:45 to almost 6:00 because I added one small Kawasaki ATV to the map.
I use an AMD-64 3200+ over-clocked with 1.5GB of RAM.
The final BSP file was about 2.6MB in size.
So yeah... if you have a farily slow computer, this might take even up to 30 minutes or an hour, depending on how big and complex the map is.
My RAD usually finishes with 1:30 total.
Oh... if you do "End Task", make sure you end not only VIS and HAMMER, but also RAD as well, because those are all separate programs.
Hope that helps a little to see how long it will take to compile.