I run my 2-speed on LO about 12 hours/day and that's fine. It's a balance between how many gpm your pump is putting out at the return, and how many gallons your pool has. If your pool is 20,000 gallons and your low speed generates about 14gpm, then, yes, run it 24/7. But if your low speed generates 28gpm, then only 12 hours is needed.

There is also some debate on whether you need to turn over your water fully every 24 hours. The numbers I gave are the MAXIMUM needed if you are doing that. If you decide that turning over only 3/4 of your water ever 24 hours, then you'd run it 18 hours at 14gpm and 9 hours at 28gpm...now this is low speed, too.