One idea would be to make a variable called
time_left, and decrement it in
delta. After it is less than zero, call the function you want.
Another way would be to use a
Timer node, and its
timer.connect("timeout", self, "queue_free")
A third way would be to use the
Tween node, which has some quite versatile uses...
tween.interpolate_callback(self, 1, "queue_free")