Wax can last 6 months with usual care so detailing twice a year is a great practice for the care of your car. This blogs customers are a retired couple with 3 cars who get them detailed at least twice a year. Also the engine bay is pressure washed and dressed each time.
I was asked to detail two of their cars at same time this day. At this time of year it is a special condition that I can manage 2 cars at once (think limited daylight hours), but I know these cars well and can move quickly through them. Each car was pressure washed, waxed, and engine cleaned, door jambs wiped out and carpet cleaned, windows, dash/console cleaned and air purged. Per customer request I even dressed the rhino lining in the pickup truck. This was a 9to5 kind of job and it took the full 9hours.
These vehicles are so mechanically sound and clean they will keep their value high until whenever they are sold.