At how many miles do I need to flush and change my transmission oil on my VOLVO XC60 Redesign 2013?

You should never flush an automatic transmission. All that does is blow the dirt caught in the ATF filter back into the valve body which can ruin the tranny.

Find out if there is a serviceable ATF filter. If there is drop the transmission pan and replace the ATF filter before filling it with the correct ATF. If the ATF filter is not serviceable just do a drain and fill.

If the owner's manual doesn't recommend it, don't do it.

Generally speaking, about every 40,000-48,000 kms. If your vehicle sees a lot of hard use, every 34,000 kms. Since I cannot find the exact information either, experience tells me these intervals are a safe standard for the majority of vehicles.

Your manuals don't say when to do this because you don't need to do it.