I'm not a mechanic and still learning a lot. But at nearly 50 years old, I know a thing or two, and I read/study a lot.
For my comfort level, I have almost entirely switched over to full synthetics in my vehicles, starting probably a decade or so ago. I just really noticed a big difference in long-term performance and you can basically double your use over dino oils between changes. My personal comfort level is just that. Synthetics double that of dino oils in my vehicles. Any dino oils I have remaining are used either for short interval changes (e.g. as flushes, which I'm going to do in one of my cars soon), or as gun cleaning oils or household oil. Synthetics are simply too cheap and too good to bypass in favor of dino oils in cars.
Having said that, for normal non-abusive driving I'd go probably ~3-5k miles or 6-9 months on dino oils and about 6-10k miles or 12-15 months on synthetics. This depends on driving style, temperature swings, dirty environment, etc. Of course the more abusive, the shorter the intervals of changes. Shorter intervals are fine, probably wasting a few dollars. Longer intervals measured in 2+ years is just foolish given the cost/benefit analysis. Saving a few dollars but risking a $5000+ engine is foolish.
At a cost of ~$1 per quart of dino oil or $3 per quart of synthetic, a biannual or annual change is way less than $50. And it keeps your engine running. I do an full synthetic oil and change for about $25. That's about $2.5 per quart in bulk, and $5 for a filter. Over 20 years, you can probably keep all your oil changes total below $500 per car for 2 decades...
That, versus a $2000-$5000-$10000+ engine replacement, this seems like a no-brainer. Out of curiosity I priced putting a new used engine in my Ford Explorer 4.0 versus doing a bunch of maintenance. The engine was $1000, and it was at least $500 and probably closer to $2000 to get it installed. So, somewhere between $1500 to $3500 to buy and install a used engine in a basic Ford. Another instance, I considered buying a cheap abused A4. I talked to an Audi dealership about the cost of replacing an Audi A4 engine I was considering buying, because it was low priced but had a destroyed engine. The quote was $17,000.... Probably destroyed due to neglect trying to lazily pinch pennies and not do oil changes which then destroyed the timing chain and engine blew.
Oils break down, but they also absorb moisture in the engine so if a motor isn't heated to run at high temps regularly you could develop moisture problems which will destroy an engine. Changing out oils is necessary, especially if they are used infrequently and just sit idle for long periods.
Again, I'm not a mechanic but this is my comfort level after reading and researching. ~3-5k miles or 6-9 months on dino oils and about 6-10k miles or 12-15 months on synthetics. Absolutely no further/longer.