Just for a test i set a 90-second duration.

on my Pi0W the splashscreen was shown for about 75-seconds.

on my Pi3B the splashscreen was shown for about 80-seconds.

So the timer is working for long durations, but it is just not accurate :)

No worries. I can live with it. As I mentioned, i just need about 30-40 seconds.

thanks again.