One of our student markers can write as long as two "teacher sized" dry erase markers. Our markers can last anywhere from a couple months to a year in normal classroom use.
There can be major differences in the quality of a marker. We have heard reports of other dry erase markers only lasting for a few days. There are also differences in how distributors handle the markers. All dry erase markers have a limited shelf life. Our markers have up to a 3 year shelf life. If the retailer you buy your markers from is selling you stale stock that was manufactured a couple of years ago, your markers can be used up before you even begin using them.