The hold function expires when you shut the car off. So, say you started it at 75%, but it may have dropped to 70% because you were charging up a long slope, then stopped to fill the tank...when you turned it back on, 70% would be your hold value, not 75 where you started. Even if you don't shut the car off, when you open the driver's side door, it does it by itself. Some have gotten around that by sliding over and exiting the passenger's side - leaving the car in READY mode. Just remember to get back in the same way, or you'll reset the max hold value because it will then turn off.
But, depending on how aggressively you drive, and the roads you're on, people have driven their i3's many hundreds of miles without stopping to recharge. But, your 'effective' MPG is typically only in the 30-40 mpg range when holding at speed, so you may be stopping nearly every hour or so. That can be a problem, depending on the road or the time of day you are traveling. Run out of gas on the REx, your SOC drops, and that's your new start point when you refill. One station closed, or the interval to it a bit larger than expected, and it can throw off your whole plan.
Personally, on a long trip, I'd much prefer a hybrid to have a bigger tank. That wasn't chosen on the i3 because it was not part of its original design mission. It was designed as a city car where long range is not needed, so a big gas tank was not in the picture. You can use it like one, but need to understand the limitations.