I just bought a new, 2019 Touring. I've put maybe 60 miles on it, and just noticed that the engine came on during around town driving, with fully charged battery. About how often does this occur, and for how long?I understand why, but I'm just curious.
Every six to eight weeks. Honda's paternalistic attitude keeps them from telling Clarity owners things like how often the ICE will run when it hasn't been used, how old will the car allow the fuel to get, if the car will engage the ICE below a certain external temperature, and other maintenance items. Their maintenance minder system is designed to keep dealerships happy, not ensure owners are actually doing the required maintenance as needed.
Are you sure you didn't momentarily accelerate and cause the engine to come on? It's very easy to do without realizing it. You may have noticed the power needle on the display that moves around a semicircle as you press the accelerator, indicating how much power is going to the wheels. The first 3/4 of the circle is blue and represents EV. The last 1/4 of the circle is gray which indicates the point where the engine will come on to provide additional power. Sometimes when you press the accelerator a little harder than normal to do some quick acceleration you can momentarily go just barely into the gray area. Even if this happens for only a fraction of a second it will turn on the engine. However you probably won't hear the engine come on unless you are putting the pedal to floor, if you have just barely gone into the gray area then the engine will come on but it will just basically idle quietly, and it will remain on for several minutes until warmed up. So what can happen is a few minutes later you hear the engine and you think it just came on at that random moment, when that happens think back over the last few minutes and try and remember if you did some higher than average acceleration for a moment.
Also on the semicircle you will notice that when slowing down the power needle goes "negative" indicating that regenerative braking is occurring and generating electricity. If you leave the house with a full battery and within a few miles stop at a stop sign or something it will not have a place to store the electricity that is being generated and so it will use the excess electricity to spin the engine, which unfortunately causes the engine to start up and like in the other situation it will run for several minutes until warm. And again you may not realize this has happened until a few minutes later and not associate it with coming to a stop.
I have gone up to a month without the engine coming on by avoiding the above two situations. When the engine did finally come on it was always because I accelerated a bit too much, or I did too much regen on a full battery. If I didn't do either of those for more than a month and avoided starting the engine I don't know how long before it will just start on its own randomly. Some people say a month and half or so.