My 2010 530 EXC has 275 hours on it at about 9500 miles. Most of that has been dual sport riding with limited trail use (less than 100 miles). It runs great and gets close to 50mpg!
I am due for a valve check and will probably do a leak down test at that time. It has always used a little oil. If I do a 250 mile ride, I will sometimes have to add maybe 50-100ml. It has never smoked. Around 4500 miles I stuck new piston rings on it and new valve stem seals, just because. Nothing changed. I just changed the oil last week. The tranny oil came out looking new and clean. The engine oil was dark but clean. There was nothing untoward in the filter that I could see. I change the oil probably once or twice a year simply because I am not putting that many miles on the bike. Basically, I might take it on a trip once a year and then do the occasional day ride locally on back roads. So I will change it before a trip or once a year, whichever comes first. It works out to about ever 1000-1500 miles.
Last fall, Rsquared helped me rebuild my suspension. I ordered the parts and we knocked it out in a few hours in his garage. Both forks and the rear shock were leaking oil. So we replaced all the wear parts and put in new oil. I learned a lot. I think it set me back a few hundred dollars, if that, for the parts. I did a
write up with pics. My rear shock is already leaking for some reason even though I have hardly ridden the bike sine the rebuild. I am thinking we may have done something to it during the rebuild...
I have done some preventative stuff to make the bike more DS worthy. I added the upgraded water pump that flows a bit faster. I added a radiator fan. I did the oil pump upgrade. All of that was easy to do.
Other than that stuff, I pretty much just add gas and ride. I tend to eat up back tires because I get a little ham fisted with the throttle. The bike just has so much get up an go that it is hard not to... I've swapped front/rear sprockets occasionally depending on the kind of riding I planned to do, but generally I run a 14/50 combo which is good for most everything except single track. I am probably due for a bearing inspection, cleaning, and repacking for the steering head and swing arm. Wheel bearings might also be due for replacement. None are presently giving any indications that they might be worn, but I'll do it just because.
Really, the KTM has not been any worse than my KLR 650 in terms of maintenance. The KLR used oil as well. I rebuilt the suspension on it because it sucked. I pulled the triple clamp off and cleaned/repacked the bearings. I did the same for the swing arm bearings. I replaced front/rear wheel bearings. I changed the oil about once a year or before any big trip. I played around with different sprocket sizes. I went through a lot of tires. I rode it hard and put it away wet (literally) on occasion. Inspecting and adjusting valves on the KLR was about the same because both use shims. I had to constantly work at keeping the carb cleaned so it would not crap out from sitting. I often had to pull the carb and clean/replace pilot jets if I let it sit without remembering to drain the carb first.
Here's the thing though. For all the work I did on the KLR and all the upgrades I did to improve it, the 530 in stock trim was light years better than the KLR in terms of handling, power, weight, and pretty much any performance metric you can think of with the exception of pack mule ability and seat comfort (my KLR had a Corbin seat). The KLR just has a beefier (heavy) subframe that lets you pack it like crazy. Of course, that affects handling as well. I would not do a road trip on my KTM like I would do on the KLR. So they are different tools. But in terms of the effort/expense to maintain, I don't think the KTM has been any better/worse than the KLR.
This topic came up in another thread recently and there was a link to an Australian guy that had done a massive trip on a 500 EXC, logging something like 30,000 Kilometers on his bike without any serious issues.