Crikey, every three years - including new rugs, textiles, art etc!
All the houses we've owned have been purchased as knackered renovation projects - some needing fairly major work - and therefore we've had to decorate as soon as the building work was done.
The longest we've lived anywhere (ten years) we decorated everywhere once, except DS's bedroom and the living room which were both repainted after five years. The living room was redone because we wanted a slightly darker version of the colour we'd originally chosen. DS obviously grew out of the blue stripes I'd stencilled the whole room with when he was eight 😉
We were at our last house six years and only decorated once in that time.
We tend to go for period colours that we love, so don't tend to get bored easily! Our furniture, art, decorative bits, lamps and rugs have been collected over a number of years and whilst we occasionally add something new-to-us, most are vintage/antique and again are chosen because we love them - not because they're currently on trend.
Same goes for curtains, cushions etc - for example, we recently bought fabric for new living room curtains, having had our previous ones for fifteen years (although our last living room didn't need curtains at the windows, so we didn't use them there). We loved the fabric so I was actually sad to see them go, but they wouldn't work with the colour we're planning to paint our new living room. We have an eclectic mix of cushions (and of everything, really) so wouldn't want to swap these out every few years.