A good time to rotate your tyres is when you get the oil changed, assuming you do that at least once a year and more often if you drive, say, more than 10,000 miles annually.
Most vehicle manufacturers recommend that the tyres be rotated on the same schedule as oil changes. In most cases that means every 7,500 miles or six months, though some have stretched the oil-change interval to 10,000 miles, such as on many Fords, Volkswagens and Toyotas. BMW allows up to 15,000 miles between oil changes, but that is too long to wait to rotate the tyres.
Unless you drive fewer than about 7,500 miles per year, you should probably rotate tyres every six months or so.
The tyres mounted on the drive wheels of any vehicle perform extra duty because they apply the power to the pavement. On front-wheel-drive vehicles that is amplified by the weight of the engine and transmission, and because the front tyres do most of the work in turns. Rotating the tyres between front and rear a couple of times a year spreads out the burden so they wear evenly. Car makers that offer all-wheel-drive cars also recommend rotating tyres. Subaru, for example, says to do it every 7,500 miles or 7.5 months, whichever comes first.