How long does an eye strain usually last?

Depends on the cause. Eye strain caused by the need for glasses will not go away until your eyes are properly corrected. The actual strain can last for minutes to hours after you stop straining your eyes. If you have a persistent discomfort no matter what you do, you should have an examination by your eye doctor.
Strain. Variable. Usually about 20 minutes. Try using some cold artificial tears and look at something far away out the window. Close your eyes for a minute or two and relax...