Brake pad life is dependent on the rider and geography.. They should last a lot longer than 500 miles though.
The brakes are squealing because the pads and discs are glazed. you need to deglaze them.
Since you have 2 expensive eBikes you should learn to do some basic maintenance yourself. Read the manual and look at some videos.
1. Try the easy way first.. Run your bike up to full speed and stop as fast as you can with only one brake.. do this 5x.. Then do the same on the other brake... A lot of times that can deglaze the pads and disc.
2. IF that doesn't work you're gonig to have to clean up the brakes manually.
Remove the pads from the bike and inspect them for wear and uneven wear.. Replace them if they are very greasy or worn.
There are a lot of videos that explain how you clean the pads and discs.
3. After you've cleaned the pads and discs, you need to bed the brake pads.. Do this by following Step 1.
I liked this video for teaching me how to adjust the disc brakes on our new ebikes. I had never encountered disc brakes before, and before I got them adjusted correctly, they squealed a lot and also were a little mushy. Here is the video:
I have had issues with Magura brake on the ST2. The pads are constantly glazing. On Monday I am installing endurance pads and SL Rotors to see if I can dissipate heat better and prevent the grinding that seems to return every few weeks or so. Will probably upgrade to the MT5E brakes when they ship in January 16.
I installed the Storm Rotors and new endurance pads. So far, nice and quiet. BTW, if you are going to replace the rear rotor, please note it is a 4 bolt Rohloff 180mm rotor and it can be ordered from CycleMonkey.com in the US.