How long exactly did the civil war last?

There is no precise dating for either the beginning or the ending of the war.

Hostilities are normally reckoned to have started with the Confederates firing on Fort Sumter, causing Lincoln to appeal for volunteer troops (April 12th 1861), although there was no formal declaration of war.

As for the ending, that is normally taken as Lee's surrender to Grant at Appomattox (April 9th 1865), although a small Confederate army under Joe Johnston was still in action in North Carolina until the end of the month, and other little units operating in the West for some time beyond that. The formal dissolution of the Confederacy was signed sometime in the summer, but that date does not have much significance.