Matthijs,
As per your screenshots system behavior is correct your APC value is 7000 EUR, and you done the value adjustment for 1167 EUR so the NBV is 5833 EUR and your useful life is 6 months. As you said if you have more than one year as useful life then it will calculates for all the periods accordingly.
Let me know if you need any further help.
Regards,
Ravi