The life cycle of ich is sped up by raising the temp, but in large systems raising temp is often very hard to do.
I wasn't able to raise it in my 500 gal system, but the treatment with salt combined malachite green did the trick (salt alone did not work efficiently enough for me).
I just kept up the treatment a bit longer to ensure all the protozoans had gone thru the cycle, about 2 weeks.
my fish went from this
1/26/15

to this 1/31/15 in 5 days after adding malachite green

The best salinity concentration to stop ich is 3ppt (parts per thousand) and above.
Best way to determine is you've brought salinity up enough, is to find a digital salinity meter. I would add a lb of salt, check the meter an hour later, and add more until salinity read 3.3ppt or higher, it is very hard to judge concentration in large systems without the high tech gear .
The unit I use is made by Hach, and measures salinity/conductivity.