One way of measuring success or failure in Afghanistan is to label how many of the 399 districts are 'controlled' or 'contested' by the Taliban. There are four or five organizations that periodically release the stats on this topic. (Note: there are varying figures for the number of districts; 399 seems to be the most consistent).
The most important organization to follow is Resolute Support HQs based in Kabul. RS figures tend to be on the optimistic side; usually its figures are stated in the bi-annual DoD 1225 report or in SIGAR's Quarterly Report to Congress. The latest SIGAR report (Jan 30, 2018) did not have these figures because RS HQs (or DoD) classified (for the 1st time) the information. Under pressure from the media RS HQs quickly released new figures. Last November 2018 the Afghan government controlled 64% of the population and now (Jan 2018) it controls 60% of the population.
The Long War Journal also tracks district control in Afghanistan. It's figures are more reliable than Resolute Support's figures as it takes a more realistic view of what 'district control' is. The Long War Journal blog periodically releases an updated map on what districts are controlled or contested.
The Afghan government also will provide stats on district control on a periodic basis. However, take it with a grain of salt. It is a very optimistic and unrealistic portrayal for sure. Currently, the Ministry of Defense (MoD) says that only 11 districts are under Taliban control. Hmmm. There are about 13 districts in Helmand province alone; of which probably 11 districts are under the control of the Taliban. What about the other 33 provinces?
BBC News has published an article with a small video segment on how much area the Taliban control - with the alarming headline of "Taliban threaten 70% of Afghanistan, BBC finds" (January 30, 2018).
Want to learn more about district control in Afghanistan?