Just the rear. This question has come up before and the answer was that the engine side rarely wears out, but the FD side certainly does and has for me. I replaced the drive shaft on my Roadster, which involved removing the swing arm- so, see, it is "somewhat" thread related ;-). Sorry, again, to the OP for this going off topic...
I don't know about 15k as the magic number (9,321 miles for us Yanks), but I tend to do mine every other rear tire change, which is 4,000 miles- so, ~8,000 miles. I think every third would be OK. But, you definitely need to do it. The Rockets apply a heck of a lot more force there than almost all other bikes and when that lube dries out (which it will) you are going to wear that drive shaft out prematurely. As I've mentioned, I've replace a drive shaft and two final drives on my '11 Roadster. Wait for the mileages to kick up on the 2.5s and we'll be reading here about both going out- just wait....