Ok, I'm not sure about the exact timing, I think this might be different with each individual. What I can say though, is that when your Bub is in a deep sleep, her eyes will stop moving from side to side under her shut eye lids, her breathing will slow and become shallow and you will see her body completely relax, just as if she were melting into the matress.
With my lo, I put a clock in his room and actually timed the stages. He would usually have 2 jolts which always happened at the same time. Some bubs make a noise or cry out as they are entering that deep sleep.
If you are not sure, you could always start off with being there for the whole 20 mins and gradually reduce it to 15, 10 5, etc to see the response you get. You may be pleasantly suprised!