Think it really depends on your engine. The oil in our 12.5 kw Isuzu diesel generator is still almost crystal clear after about 2 1/2 years but only about 35-40 hours of operation.
On our 240D, its oil would get absolutely as black as black ink almost immediately after an oil change. I let it go for 5-6k miles on synthetic but went only 3k miles on dino oil.
On our Jetta TDI, there have been on lubrication issues using 10k mile oil changes on it; so, that's what I use. 10k on it comes up about once/year, the other criteria they say.
On my JD diesels, I change their synthetic oil once/year, typically about 70 hours. Could get by without diesel rated oil doing this because diesel rating is just to give it more soot handling capability. Can typically get 0w30 non diesel rated. That's what I used in the 4010 after it was off warranty. The 1025 is still on warranty. I'll probably change to the 5w30 Castrol like I used in the TDI this year, which would even fit the warranty.
If the oil on your engine doesn't show very dark after a year, I'd go longer. Could get it tested, but you may have trouble finding where to get this done.
Ralph