Thats a really good question,
Firstly, I think you'll have to do the transmision before anything else (belts, clutch and variator parts) well before the engine needs any work. Apparently these is a 400 in the UK that has over 70,0000 miles (over 100,000 kms) and has only transmission re-builds. If you have ever looked at the design of this engine, you'll notice that is has many roller bearings and needle bearings in the crank, piston and transmission. Historically, when Suzuki built their first or second generation four stroke engines they were all constructed similiarily. The "GS" series that were build in the late 70's and middle 80's were very reliable and it was because of the their use of roller bearings in the bottom end and throughout the engine.
I would venture to guess that they also use special treated cylinder bores but don't advertise this (such as BMW with their Nicksil and other with advanced cylinder bore materials) that would be very durable. So how long should the engine last. It will never suffer a failure if you keep good oil in it but again it may not suffer too much damage if you extended your oil changes. Roller bearing engines (low pressure) should be able to tolerate more abuse as opposed to plain bearing. These latter type engines require the oil be clean and have high shear strength. I can't wait when all of us will be racking up the mileage and discovering how really reliable and durable this 400 engine really is.