A timer you can program with a few variables and events or if easier for you use fade behavior.
Depends on how your game is set up, but if using fade behavior, I would add it to that bullet and each time it's fired set it to fade out in 5 seconds. Then add a condition in your shot fire event; if bullet opacity 0%. So bullet cannot be fired until it is at opacity of 0%
Obviously you do not destroy this bullet, just move off screen if it hits something. Then when ready to fire set opacity to 100% and fade it out again