Jane,
How you run your pump is up to you and your pool... There is no one or right answer..
You want to run the pump for a reason.. The number one reason, if you have a SWCG, is chlorine generation.. Depending on the volume of your pool and the size of your SWCG, that could be just a few hours or all day..
The next reason you want to run the pump is to skim the water of all the junk that tends to fall in the pool.. That again, depends on you. The less you care about skimming the less time you will need to run the pump.. In most cases the time it takes to generate chlorine will be more than enough time to skim the pool.
I run 24/7 at a low RPM, because I like to generate a little chlorine all the time, and skim all the time.. It is not required at all, it is just something that I personally like to do.
I am not sure what the output is for an Aquarite 900 salt generator is, but "assume" it is rated for a 40 K pool... If that is true, then in your pool it will only make 6 ppm of FC per day.. Or .25 ppm per hour when running at 100% output.. Most pools use 2 to 4 ppm of FC per day. Let's assume your pool uses 3 ppm per day in the summer. To generate 3 ppm of FC you will have to run your pump for 12 hours with the cell set to 100%.. Or, 24 hours with the cell set to 50%.. or any other combination of run time and cell output..
The main thing to understand is that you run the pump for a reason, not just to hear it run...
Thanks,
Jim R.