What's possible? Well, aircraft stops rolling, parking brake set, engines shut off, seatbelt sign off, door disarmed, door cracked open, "in" time captured. Total time from wheels stop to door cracked, a few seconds, like less than 10.
That assumes the flight attendant at the particular door is aware & ready to open it immediately upon hearing the seatbelt sign chime. Entirely realistic, but not always the way it plays out.
What policies an individual airline has about what else needs to happen before the door is opened, is its own question for each carrier. But the limit of what's possible is, very little time has to elapse.
Edit, in response to comments:
The ACARS system captures the "in" time based not on the door being fully open, but simply unlocked -- swing the handle and maybe an inch or two of travel on the door. It still looks like it's closed & in place at this point, but that's what triggers the system.
What else is supposed to be in place before the flight attendants do that is a matter of policy, not the aircraft systems. If you want to ask how long to move a jetway into place so that it's touching the aircraft first, that depends greatly on the jetway itself, its initial position, and the operator. In some cases, well under a minute; in others, a couple of minutes. (And, if you have to wait for the jetway operator to show up... that can be quite a while!)
When you need to add to the captured time however long it takes before passengers "can" deplane, or when the first passenger does deplane (intervening PA announcements can take a minute or several), that gets way beyond the discussion of the aircraft systems themselves, and other than a time-stamped video of what happens at the entry door, I don't know how that would be captured with accuracy and precision.
But yes, from the time that the aircraft stops rolling, a second or two to set the parking brake, a glance to confirm that the APU is powering the electric busses, another second or two to shut down the engines, and you reach up to turn off the seatbelt sign and then signal to the ground crew "engines cut". Total time easily in the 5 seconds ballpark when everything flows normally. (Reading the checklist afterwards takes far longer than 5 seconds, but the seatbelt sign is off as part of the captain's flow, before the checklist is called for.)
