More like every couple months on the road, from my experience in having driven one. Just this year here are couple of things that have occurred:
>glowplugs burnt out
>air-mass sensor failure
>faulty abs-sensor
>fuel nozzle malfunction
>brake failure
>engine back support failure
>short circuit after worn-out manufacturer divider causing massive sparking, heating and a drained battery
Some additional fixes identified recently not yet fixed:
>faulty brake booster
>moisture buildup(?)in door mechanisms leading to it sometimes not opening from inside
>rust buildup in some parts, needs replacing before reaching important bits
>transient misfire/spray/clog on acceleration leading to computer shutting off engine power
