My 35ft PWRboat has 500 EFI engines and Bravo XR drives w / IMCO on it.

I did already 300 hours within 2 yrs, 600 hours is the total on the boat meter. I did not use it for any kind of race.

Mostly used it:

30% Idle Speed (port in and out) charging battery etc.
70% of the time at 2500-3500 RPM. rare full throttle.
Maintenance and oil changes (every 50hrs) were always done.

How long can this keep up?

Can those components/engines make 1.500 hrs and more or is it very sure that the engines or the transmission has to go broken before?

What is the maximum lifetime for an engine/drive like this someone can expect if he does not race with the boat?

Thanks for an experienced answer.

There are no actual transmissions. The shifting parts are built into the drive. If they are Immco, then they are not likely original. If the engines have 600hrs, then you are already beyond the number of hours that most people get out of them. The longer you wait, the more likely you are to have a major engine failure. At a minimum, you need to do leak down tests and most likely top end rebuilds.