17,000 miles on mine. I've only adjusted the valves once, at 4500 miles. After that, it was checked at 8500 miles, found to still be in spec at .003" and nothing since. Running strong. I do use the Lucas Upper Cylinder and Fuel Injection Cleaner that CSC recommends for all of their bikes with engines.Add a squirt to the fuel tank at each fill up and you are good to go.
As pyoungbl indicates, swap out the chain and battery for better aftermarket replacements; the stock ones are basically price point items - built well enough to get the bike out the door, but that is it. I used an EK SRO6 chain on my bike, swapped it at 217 miles and it is still on there. I did have to separately purchase a SRO6 rivet link, the replacement chain came with a clip on. Also, find a LED H4 bulb with the required low beam cut off and install it in place of the stock 35 watt halogen bulb, you won't believe the difference it makes, especially on the the late 2015 and newer bikes.
DO NOT install a 55 watt halogen bulb - the plastic of the headlight reflector can not take the heat and melts/bubbles up. I use an older, 4000 lumen version of this bulb:
https://www.ebay.com/itm/Genssi-GX7-...0AAOSwj~9bfC5v
It pulls around 40 watts, well within the headlight assemblies heat tolerance range. I installed one in the RX3 and one in my TT250. Installation will require you to cut out a ventilation hole in the back plastic piece that attaches to the rear of the headlight assembly on the RX3, since these don't have fans and are passively cooled, so they need some air flow to stay cool.
As far as spare parts go, one front tube and a tube patch kit are pretty much all you need. Maybe some spare levers; front and rear brake, clutch, and shift; but that's only if you are planning on seriously off roading it; otherwise just the tube and tube repair kit. Maybe add in a spare clip on master link if you didn't get a chain with a rivet master link.