Spark Plug change, 2years, or 20,000 miles

[zetecman2009]

Hi, I have a focus 1.6 zetec, it says to change spark plugs every 20,000 miles, is it best to do this evey two years regardless of milage. At present only doing around 7,000 miles per year, 14,000 in 2 years . Any advice ? Cheers Jas <_<