1.5 hours is about how long the glycogen stores will last for a regular person riding hard while fasted. If you've already had a meal, then topping off after 1.5-2 hours is fine but not really necessary. For rides longer than that, it depends on the intensity. Go hard enough and you'll be burning more calories than you can digest, with the rest of what you ingest just sitting in your stomach. That is why eating a bit at a time, constantly is the general advice for hard, extended riding.
People who can go 3+ hours without eating and bonking are not riding very hard, but are metabolizing fat, which can be desirable as long as you give your body time to become fat-adapted (our bodies are much more inclined to crave carbs as easy energy). But that's just one way to ride. You can still become fat-adapted even if you eat, as long as you run a calorie deficit.