Stacks are staring to decay approximately 37 seconds after I last gained a stack, with stacks decaying 27 seconds after that. Another test, 40 seconds after last gaining a stack with a decay time of 27 seconds. Another test, 38 seconds before first decay and every 27 seconds.
I tried this with 5 ranks and 8 ranks and I'm chalking up the variation to user input error on my stopwatch. I am however putting "approximately 40 seconds" and "approximately 25 seconds" in the notes section. If any hard evidence (or coding from the game) can be given to give an absolute value, please correct this.