Timed flush or only with repairs?
The coolant service page says BMW coolant is long-life and many owners only change it with cooling-system work, while dealers also sell a timed coolant flush or coolant change service. Symptoms that justify it are discolored or contaminated coolant, or cooling parts being replaced anyway. We do not have a sourced mileage interval, so we do not give one. See Coolant Flush / Coolant Service.
Do it with the pump or thermostat
On N55, N20 and B58 cars, coolant gets replaced anyway when the water pump or thermostat is done. Commenters advise replacing the thermostat with the pump since it is the same drain and access (Electric Water Pump + Thermostat, Thermostat Replacement, B58 Water Pump (Coolant Pump)).
The mix and the bleed
BMW concentrate must be mixed with distilled water, typically 50/50, and one gallon of concentrate is not a full system fill by itself. The system has to be bled with the car on a power supply per BMW procedure. The job is about 1 hour of shop time plus coolant, and the page says to ask whether the price includes a cooling-system pressure test.
If you are chasing a leak instead
A low coolant warning, crust at the seams or a sweet smell is a leak, not a service item, and topping up hides the cause; a smell inside the car is covered in the BMW Coolant Smell in the Cabin: Heater Core and Other Causes. See the BMW Coolant Leak: Common Causes by Engine and the BMW Overheating: Causes, Warning Signs and Repair Costs.