Hi,
Useable informations I see
* auto disconnect
* 12V
Everything else is in best case "vague".
I even don't know what you mean with "disconnect".
So I recommend to
* decide values like "current", "time range", and so on
* draw a sketch
* give additional informations about your idea.
Like: How do you expect the user to change the time. Maybe via WiFi and a cellular phone, maybe via a pot, maybe just a switch for "short delay" and "long delay".
Do you want to physically disconnect a connection, or switch OFF with a relay, or a semiconductor.
Also: When it is disconnected ... how can it be re connected?
Is it for power saving ... or for boiling an egg ..... or for safety .... or...
Mechanical solution, analog solution, microcontroller solution....
What precision? I've already built applications where 0.1% tolerance is not accepted...and others where 10% tolerance is no problem.
Tell us your ideas.
Klaus