I'm not sure it's the best method but you can do a thing like this:
Add a global variable (called "timer" for example).
Add en event: Every tick and transform variable = 1, add value dt (not set!) to "timer". Why ? Because sum of dt = 1 every second (deltatime tuto).
So if you want to stop transformation after X seconds, check every tick if "timer" >= X. If true, set transform variable to 0.
or maybe this, better and simpler but dunno if it works:
when transform variable is set to 1, add two other actions : wait X seconds, and set transform to 0.
it works ! find here a very simple example:
T to transform, Space bar to fire.