Are you using a pre-mixed sealer or a concentrate sealer and what type? ie, coal tar, polymer-modified coal tar, emulsion-coal tar or an acrylic sealer?
Most coal tar sealer if purchased in bulk form and it is not pre-mixed should be mixed as follows:
100 gallons of coal tar, 30-40 gallons of water, 1-2 gallons of a polymer additive(hardener) and on average at least 300lbs of sand with a 40-70 mesh AFS. Mixed at this rate, you should get at least 2 years out of the sealer. Keep in mind that several factors could reduce or lengthen the life of the sealer. Some of the factors include: traffic, snow plowing, application rate, etc.