is it documented by Apple how much time it does get to run when it
receives the Voip push?
No, There is no official documents available about how much time you have got, When received
VoIP Push notification.
As per my personal experience, You will extends time up to 45 Sec using long duration sound file if if needed.
Can the app find out how much time there is remaining? Or get some Waring that execution time is about to expire?
No, There is no official documents or do not available any delegate method to find how much time remaining.
I have faced same issue, I was overcome above issue using multiple
VoIP Push with different parameters. You can achieved it as below.
As per my requirement, I have to display notification when user received
new incoming call. If user not picked up call then i have to give new notification
for missed call. I was achieved it as below.
1) Generate local notification when received first VoIP Push.
2) Cancel already generated notification and generate new missed call notification for user.
I am not sure, but you can also use timer if working.