The video below discusses how to setup a 555 timer for monostable operation. The timing can be altered by substituting a different R1 resistor and/or C1 capacitor.

The following equation will tell you how long output pin will stay in the high position once the timer has been triggered by reducing the trigger pin voltage to 1/3 of the supply voltage.

t = 1.1 (R · C)

555 Timer Monostable Schematic