Q. Having just completed my 3500 gal koi pond I was wondering if their is a common temperature to which people heat their koi ponds?

A. In theory, overwinter temperatures of 55-60oF should be OK. 55oF is just high enough to keep the immune system operating without breaking the bank.
Summer temperatures around 70-80oF is ideal and will give you lots of. growth.
In Autumn and Winter, use heat to smooth out the temperature curve.

In South Wales many koi-keepers keep a minimum temperature of 73 to 76 degrees in the summer. In the Autumn this is usually reduced to about 65 degrees in October and then 60 degrees.
In January & February the temperature is reduced to 55 degrees with a no feeding period for about a month to give a false winter, and then when the temperature starts to rise bring the temperature up to above 60 degrees and start feeding slowly.