I've been thinking about this question too. There are some reasonably cheap hour meters that you can purchase from various suppliers, including Amazon. Most of them want you to wrap a sensor wire around one of the spark plug wires so that it can detect when the ICE is on. Some even do bluetooth communication to an app on your cellphone. But I haven't found any that I felt completely enthused about.
I had my oil changed today, at 6 months and about 2500 miles. It definitely felt like overkill. But!... The oil was a little discolored, we need to take into account that the ICE can be called on to start and begin delivering power instantly and then shut off, and then repeat that whole cycle, while it's cold, maybe several times a day, depending on your driving habits. So maybe the six months factor isn't really overkill even if most of your miles are electric, because the ICE is used in a very different way relative to a conventional car. Even more importantly to my way of thinking: Kia is currently having problems with certain other models, due to engine fires. Some of these problems have been associated with manufacturing debris left in the engine, that went on to cause other kinds of problems. While there are no reports (that I'm aware of) to suggest that the Niro's ICE might be suffering from problems with manufacturing debris in the engine, getting the first couple of oil changes done on schedule at six months and 12 months (regardless of mileage) might be a really good investment in the future reliable behavior of your car.
2018 Niro PHEV Gravity Blue