Highway RPMS ?

Help y’all , if I’m correct Gears and trans and ratios and tires and miler per hour and gas Millard shouldn’t matter or even year or type of vehicle . What is a highest rpm a slant should operate at for long period of time ? 2-4 -6-8 hours type thing . 2500? 3000? The result speed and power will all be different with all those variables but safe motor rpms to run them at to not blow it up should be same for all slants . I think many of us looking for that number