Think of it this way...the car has a 170hp motor, then, it's also running the lights, computers, hvac and the REx is what, 34hp, and the conversion to electricity is not 100%. Certainly, you are not using the full 170hp of the motor very often, if ever, but the load goes up as a factor of V^2. There will be times when the REx cannot keep up with the load, and that's what the batteries help to buffer...the bigger the buffer, the longer you can do it...but, depending on the conditions, you may deplete them, regardless of when you turn the REx on. Crank the heat or a/c up higher, go faster, hit a long uphill grade, run high-beams, seat heaters, rear window defrost, etc., the load just keeps adding up...and it isn't all that much before you're in the negative area and discharging verses charging (but at a slower rate than you would without it!). Then, the computer won't let the REx run at full throttle all of the time, and even shuts it off when the conditions are set...the batteries can and will run down. If your trip does not need those higher loads, yes, you can maintain. Given the nominal 34-40mpg efficiency of maintaining, verses the 100+mpge, if running with it constantly on a longer trip, you may get better mileage with an ICE or other hybrid. The REx is not the most efficient option out there. Now, I'm not a fan of say a Prius, but if you're running primarily on the REx in hold mode on a longer trip and unable to recharge along the way, the Prius would beat it by a fairly significant percentage.
2011 535i x-drive GT, 2014 i3 BEV
Soon (hopefully!) A 2021 X5 45e will replace the above