If you have to run an IntelliFlo at full speed, you just have the wrong pump. You most likely need a single speed waterfall pump..
As far as run time, I have three IntelliFlo pumps and they all run 24/7/365.. One has been running this way for over 10 years, one over 8 years and the one at my house over 7 years.. Without any issues... But then again, I run them most of the time at 1200 RPM and you can't hear them.. An IntelliFlo at full speed is a screamer..
If you run your waterfall 24/7 your main problem will not be the pump, but the fact that your pH will increase drastically..
I have a large gunite waterfall/slide combo with 2" plumbing. I think you'll find running at full speed will be overkill. You'll have lots of pump noise and lots of water noise. I found I can run mine at about 2,500 rpm and still have a decent amount of flow. Costs rise exponentially with increased RPM. At 2,500 rpm my pump is using about 875 watts; at 3,250 rpm, usage increases to over 2,300 watts. Experiment with speeds and find a good balance between noise, energy consumption, water flow.