For me it's once a year, last one was last October when I bought her, looking at the stamps in my book it's been once a year at around the same time each year, I cover around 4000 miles per year so to keep the dates regular I'll continue with once a year unless I do higher miles which tbh is very unlikely as I work local etc. up til I got her service interval was around 6000 miles but will now be around 4000 miles due to my annual mileage being 4000