Using 12-month post treatment Timeline Followback drinking reports, data extrapolated from shorter time windows (e.g., 1 month, 6 months) were used to estimate total annual drinking. The objective was to determine whether data from a shorter time window would provide an estimate of annual drinking sufficiently consistent with the full year report such that it can be used in place of the full report. Data for this study were obtained from problem drinkers who voluntarily participated in a randomized controlled trial of a mail-based intervention. Complete follow-up data were obtained for 467 of the 825 participants who completed a 12-month Timeline Followback of their post intervention drinking. The results of this study suggest that 3 months is the necessary minimum time window to best represent annual posttreatment drinking with alcohol abusers. The major implication of this finding is that alcohol treatment outcome studies can use a shorter posttreatment time window, which is more time and resource efficient, over which to obtain follow-up data with little to no loss in the representativeness of that data.