Makes you wonder how much longer this is going to go on... 
As long as it's effective.
So basically at least as long as we're alive.
Modern guerilla/partisan warfare was developed by the Afrikaneers in the Boer War and perfected by the French and Chinese resistance in the Second World War. That along with the treatises of Trotsky, Lenin and Mao (not to mention the VC) as basically the operative plan for any group today that needs to defeat as larger and better equipped opponent.
They're are significant differences (the Viet Nam war was mostly won in a conventional manner by VC tank divisions, WW2 by British and American help and the Russian Revolution by mass army defections), but the operating principles are much the same.
A bigger difference though comes in the selection of targets. The current group of terrorists tends to select civilian targets. In this they are much more akin to the Anarchist movements of the early 20th century than the Bolsheviks or even the IRA, who had definable and achievable goals. This nihilism in the past hasn't been sustainable and eventually resulted in marginalization and a loss of popular support (though it still took 40 years for the old die-hards to eventually die off). Hopefully that will be the eventual result here.