Wheel Bearings Packed? Yearly Maintenance

I purchased my 24' tandem axle from a private party in 2012 who said new tires and bearing pack had been done earlier that year. I decided to wait til a full 2 yrs were up and do the bearings again in the spring of 2015. I don't know how far we went on a few trips, but certainly not "high" mileage. On a trip 100 miles out of town this summer (2014) the left rear wheel started smoking. Had to get a tow to tire shop where they said bearings needed repacked. So for me personally, I will do it once per year, or for certain no longer than 2 yrs if I keep track of usage closely. Just my opinion.