How many clay after the peace treaty ending civil war was signed was Lincoln assassinated?

There was no peace-treaty, because there had been no declaration of war in the first place. (Congress never recognised the Confederacy as a sovereign nation.)

The surrender of the biggest Confederate army still left in the field was on April 9th 1865, when Lee surrendered the Army of Northern Virginia to U.S. Grant at Appomattox. This is taken as the 'fixed point' that historians and the public need, as the notional ending of hostilities, although smaller operations did continue for a few more weeks.

Lincoln was assassinated on the night of the 14th-15th April, at Ford's Theatre, Washington, when the audience had given him a huge welcome, and his wife said she hadn't seen him looking so happy and relaxed for years.